I'm not a PCB design person, but using traces for ground runs instead of having a ground plane will probably have mixed results. The advantage is probably a lower overall capacitance between ground and the address and data lines. But ground traces can start to look like inductors at the higher speed switching signals modern CMOS will have. That may make it difficult to decouple the power supply for some of the chips. (You typically decouple from the supply to ground, but in this situation "ground" may not be the same for the overall circuit and the individual chip feed with a trace for ground.) I've seen this with high-speed memory chips--no matter how hard I tried to decouple it, there was noise.
It's just a gut feel, but I think that going to CMOS and removing copper from the board are both going to tend to push things closer to the edge. At the end of the day, I think the CMOS devices you are trying to drive are going to be the big contributors to the problem. (A bigger problem than any issues with the capacitance caused by the ground plane.) On the 6800 board I've been tinkering with, the 1.84 MHz oscillator has bounces (oscillation) on both transitions that is well over 10x the frequency of the crystal. In that situation, having the extra copper as a ground plane helps because it reduces the impedance from the ground pin of any IC to actual circuit ground.
The worst case always is where sometimes things work and sometimes they don't. One batch of chips might have characteristics different enough from another to make a swing in behavior. If you get a prototype built, I'd take a close look at the clock, address, and data lines with an oscilloscope to see if there is bounce/ringing. Sometimes the ringing is at such a high frequency that things seem to work because devices can't switch as fast as the ringing. If you find that, you know you have problems.
Probably Plan B would be to stick with the NMOS processor instead of going with the later CMOS ones. (Although it would be nice to have the added instruction the later versions bring.)