I used the 3.0 version of the mod and the following link could successfully grab a skin (see screenshot below). The skin is stored in "skins:files/*". (Additional Resources is used to load skins).
After updating to 4.0 it seems to be broken?

The logs state the following:

So it seems that even with full url enabled it still looks inside the "minecraft:" resource space. (While completely ignoring the inputted one).

I duplicated the skin and moved it into "minecraft:skins/*" and the link does, indeed, work fine.

Also, this is a bit off-topic - I'm not sure if this is intended or not, but after failing to fetch a skin once, it stops outputting the exception into the logs on repeat attempts to load - even after switching back and forth. This made it a bit annoying to test skin loading.