hi,
there is special technique which helps with such problems. The idea is that for your custom disassembler you have indicated all "entrance" points. For example, 0x0000, 0x0003, 0x000B etc. All these addresses are ones which you know that code executes from. Then your disassembler goes step by step through code sequences of all these points. It must remember all conditional jumps and process them too (it may be easy done with "thread queue").
Well, it may be that a disassembler comes to JMP @A+DPTR, @A+PC, or even "return-via-stack pushes". In this case it should report such troubles to you. Then you look at the code yourself and provide new points to be disassembled manually.
Finally, all bytes of program memory which were not involved as program part threads, are regarded as constant data.

I created such disassembler for Z80 core about 15 years ago for customer request (oh, it was not so easy due that hardware used IM2 interrupt mode). Anyway, I think it is not so hard to create the same thing for 8051 as well.

The only one thing: why do you need with such "kewl" disassembler? Are you try to hack a copyrighted program? In this case, please forget my words!

See you,
Oleg