You are touching an issue that I believe many of us have experienced. "there is that little chip with just the I/O we need, but our tools does not fit".
If this is your first '51 project LOOK OUT, if you are not careful you will end up using the ADuC in future applications even w/o analog in order to save tooling costs.
My approach is that if a small to moderate number of units are to be build, use a standard '51/52 and external peripherals, that keeps the tooling cost in place.
If many units are to be build, the tooling cost will be distributed so thin that it does not matter.
Then there is that last case where power consumption or size is a problem.
Although not with the ADuC, I have had to debug with one chip and run with another.
Rig your board to use whatever emulator you have - or buy the CEIBO EB-51 VERY INEXPENSIVE and 95% as capable as the wallet-draining stuff -. Make your routine so that an external interrupt picks up the value in some memory location (patchable). This will emulate the ADC input. If you have code totally debugged and working in this fashion, you should be able to slip the convert routine in and run.
Been there, done that.
Erik Malund