I've been thinking of adding a rough side out strip to the backs of the expedition sheaths, with a snap on cover piece, so you could load one (or both) sides with compound. total thickness addition would be minimal. Whatcha think?
Hi! You've just gotten every leatherworker in WSS ready to to infodump! I'm a bit more into the technical side of leather preservation than koyote girl, so I'll do our portion of the infodump:
For the most part, woven synthetics are flammable and burn in melting globs. I have had this happen (a strap, not a sheath) and it is ugly. Not napalm by any means, but not joy.
Leather can be preserved any number of ways, but basic sealing oil/wax mixtures will keep a sheath going for decades. Often longer than a cordura sheath- ESPECIALLY if handmade and fitted to the knife. (generally, this doesn't apply to kydex, which will outlast cockroaches) - leather finishes can give you everything from very supple leathers that will stretch like a glove over a knife handle to kydex-hardness wax immersions that knives will audibly 'click' into.
I have had woven synthetics mold and rot on me in humid hot environments.
Only leather and kydex, to the best of my knowledge, can be easily formed to a specific knife or gun shape. (Or any other tool, I suppose) Kydex tends to wear contact points much worse than oiled leather.
Leather is hackable. If you need to sew something on, add a Dee ring to a strap, or something, it's leather
The weird-ass final fringe of my answer:
A good leather sheath is a set of survival gear all on its own. To use one of the expedition sheaths for an example, you have one removable belt hanger strap. I don't know what you need a 9 inch piece of 1" wide leather strapping for but it's there. The horizontal straps are individually stitched and can be removed, also. The sheath itself is a source for more leather material (I can think of a few things off top of my head that i'd take apart a sheath for in the wild, and boot repair tops the list) with several feet of heavy duty waxed sewing cord available.