Making the most out of rawhide armour

This is a Frame Challenge

So i'm writing a parody LOTR/DnD/ASOIF (except with no magic) setting where the classic axe-wielding barbarians defending their forest look like ... generic muscle-bound ill-proportioned monstrosities wearing animal skins and foot long suebian knots protruding upwards from their heads. (Emphasis mine.)

You're asking for a lot of scientific detail for a parody. If you're going to parody those sources, you absolutely must look back to the 1970s. Just take a moment and gaze upon the wonders of Boris Vallejo. Click to enlarge.

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Is there anything there that suggests they had any kind of technological enhancements to improve the reality (or even the believability) that what they're wearing would stop knives or spikes?

Heck No!

And yet did we, the 1970s generation who grew up on LoTR/AD&D, happily believe that each and every barbarian looked just like that?

Heck Yes!

If you're seriously looking for help factually improving the armor capacity of barbarians then you're not writing a parody. For definitive proof I give you...

Sir Terry Pratchett's Cohen the Barbarian!

The absolute echt in you-can't-touch-this barbarianism.

Enhanced buffcoats and steel plackarts... Thppt!! Pansies.

:-)

I hereby invoke the Rule of Cool. No self-respecting barbarian would wear anything other than a loin cloth/bikini bottom (preferably made out of bear or lion skin) with an optional wide leather belt and rippling pecs or a loose-fitting chain mail brassiere for optional modesty to top it off.

I dare you to say they're underdressed or underprotected.

Lemme add one more thing. Think carefully about what you're trying to parody. When Gygax and Arneson were creating Dungeons & Dragons they didn't care at all what any character class wore. It didn't matter because, ultimately, the description had to give way to the game mechanics — if it mattered at all. Character classes were stereotypical, even when derived from sources like H/LoTR which, frankly, threw the issue of armor to the wind. All those goblins against one little hobbit with a knife and a waistcoat? Yeah... a +5 waistcoat with a +2 short sword endowed with Dweamor (I can't even remember how to spell it...). My point is that if you're going to make fun of something, make fun of the idiocy behind the mania. We're talking about a phenomena that set off a nation-wide moral panic.

If they have access to linen or other vegetal fibers, they can use something akin to linothorax to increase their protection.

The linothorax (/ˈliːnoʊθɔːræks/, from the Ancient Greek: λινοθώραξ, Ancient Greek: [linotʰɔ̌ːraːks]) is a type of upper body armor that was used throughout the ancient Mediterranean world. The term linothorax is a modern term based on the Greek λινοθώραξ, which means "wearing a breastplate of linen"

Throughout history, many cultures from India to Scotland to South America have made "linen" armour by quilting many layers of fabrics such as canvas or leather together or stuffing them with loose fibres such as cotton.

The answer isn't in their armor the answer is in their tactics and the terrain they have to fight in.

There are a number of issues in your question. Firstly no late medieval army in history ever consisted purposely of heavily armored knights! Not only because those knights needed support personnel to maintain their horses, camps, armor and weapons etc. But also because the late medieval armor you describe was fantastically expensive and only ever warn by the elites of society since they were the only ones who could afford it. Instead the vast majority of 15th century European armies consisted of massed pole arm equipped infantry wearing lighter/cheaper armor, crossbowmen or arquebus (primitive firearms) armed troops and an artillery train crude muzzle loaded cannons firing stone or iron shot and some heavily armored knights!

Secondly because given the terrain you've stated the fighting will occurring in i.e. forest (presumably with its fair share of hills, valleys and marshes & rivers etc) I cannot image a worse choice of armor for a soldier to be wearing. Put simply Gothic plate armor was NOT designed for use in close restricted terrain like the forests you've described. Instead they were designed for massed combat in open relatively open terrain. For a start the high level of protection afforded by Gothic armor came at the cost of restricted vision and hearing. A regiment of gothic armored fighters walking in formation into a forest would as individuals be unable to hear or see enough as individuals to fight effectively.

Lastly,your barbarians. They're not encumbered in heavy armor and or helmets that restrict their vision and hearing but are armed with lots of heavy (Conan the barbarian style) choppy,choppy stabby,stabby weapons designed to pierce heavy armor - what a coincidence! They also know their home and it's terrain intimately (every fold in the land every thicket, every stream and hill) while presumably their invaders do not! Also they are used to fighting in massed groups or as individuals so they'll be effective in small groups staging ambushes or in larger fights.

All of which combined means your barbarians (if they are also smart and well lead) can ambush, raid & harass your knights while only choosing to meet them in a massed battle when and where they chose to i.e when they have the advantage not theknights. A (very loose) example would be the battle of the Teutoburg Forest, the Germanic tribes and the Roman Legions who fought there.

Conclusion? If your invading army does consist entirely of gothic knights? Those knights are toast.