So basically you want to translate each hex character to a bit mask, assemble the bit masks, reverse their order and then collect all indexes for which the bit is 1?

I'm sure that pack and unpack can help you quite a bit, maybe together with vec.

(And it might make it easier if you work with pair of hex characters instead of single characters, because a pair of hex chars is a byte, and that works nicer than a 4 bit nibble for many operations).