In the process of micro-controlling synthesizers VCO's (musical synths that is) i've tried a few of the techniques mentioned and they all work. Oh and plenty with 8051 (keep on topic).

Steve's DAC setting the VRef for a following Mult DAC is interesting as you set the "size of your step" with one DAC and "which step" with the other DAC. A few famous early "wavetable" synths used this technique, and i built a DCO using this technique once.

The weighted summing of two 8 bit DAC's could very well result in 16 bit specification but both DAC's will need very good DNL as well as INL. I have held this technique to 14 bit's of range with good result in MIDI to Control Voltage processors for analog synthesizers.

Weighting DACs such that one DAC "fills in the cracks" is also very interesting and eases the DNL requirement for both DACs but extending resolution to 16 bits is a challenge.

As for arbitrary codes, never found this to be a problem as the DAC driver either used a table or fixed algorithm to generate the value issued to the DAC.

Everyday (including today) it is easier and not all that expensive to implement 14/16 bit DACs especially with the offerings avail. from Analog and TI.

regards,
p