I've thought of doing one of these for my favourite 8-bit computer, though never actually got round to actually trying it.

How wide is a bank number? Even if it's the full 8 bits, that's only 24 bits, which is nothing these days. 64 MB for the full table, assuming 32 bits is enough to cover every offset into the JIT buffer. (Assuming max 4 GB of JIT'd code. If that isn't, perhaps 40 bits would be? 128 MB table, max 1 TByte of JIT'd code.) Would you need a hash table rather than an array?