Be aware that the R/W line only goes low when the processor is executing a write instruction. If the data you are feeding to the data bus does not cause the processor to execute a write instruction, the R/W line will never go low. This may be what you are experiencing.

I did a little more poking ...

Sorry about the late reply. I verified that the DIP switch indeed works, though I'm not using any sort of gate between RWB and the switch block. Here's the best I could do as far as a schematic of the area in question:

What started two weeks ago as a CPU test board has quickly grown into this:

Image no longer available: http://rdp.maclisp.org/65C02board-5s.jpg
All has seemingly gone well, to the point of figuring out how to write to the data bus.
Seeing as RWB goes high when the processor expects to read off ...