Sorry, just a clarification...

If you have "func() using 1" then you can indeed call it from another register bank. However, this is not useful because:

(a) The function can't return a value in a register - "unsigned char func() using 1" is illegal, so this is not a general technique.

(b) The overhead of "PUSH PSW, MOV PSW, #XX" at function entry and "POP PSW" at function exit generated due to "using 1" are likely to outweigh any savings of absolute register addressing anyway.

It's when you try to use "#pragma registerbank(X)" to mark a function as using a register bank without generating entry/exit code to explicitly set the register bank, that you start to hit problems with calling function from ISR and also from main.