1) Yes, but write to that address in RAM can corrupt value(eg CLEAR instruction in bootloader). Be careful.
2) Yes and No.
Yes, it can be accessed via ASM. But that isn't ONLY way. Other is via modifier EXT. More about EXT: http://www.picbasic.co.uk/forum/showthread.php?t=3891
BLOCK_SIZE is constant, not variable. And BlockSize is best to leave to compiler to allocate variable as all other "regular variables", but in BANKA. BlockSize VAR BYTE BANKA.
I think, but not sure, that is best to leave to compiler to allocate variable. If you access lot to that variable then it should be in BANKA, or to simplify ASM code. And if you must, then you specify location.