You would need to check the compiler manual carefully for the conversion rules between different memory spaces for pointers; Keil has a whole section on it.

I think it would be very dangerous to try casting a DATA pointer to, say, a CODE pointer; normal 'C' casting is just about different types of data; with 8051 pointers, the data could also be in totally different parts of the physical architecture!
eg, you can't write to something in CODE space - no matter how you try to cast it!