If we're trying to fit everything into a functional 'cycle of 5ths' system, IV is actually a substitute for ii, V of V. ♭VII 'works' because it has two notes in common with ii.
♭VII7? Well, in a jazz/blues environment, you can add a 7th to just about any chord and it won't sound bad!
There are similarities with the 'backdoor progression' where ♭VII is used as a substitute for V7. Again, it works because ♭VII has two notes in common with V7.
But beware of trying to hook EVERY progression onto an underlying functional ii, V, I. 'Giant Steps' took that sort of thing about as far as it could go.