Hi Stevie.
There is a workaround by using Colour Management built into PM. There are B&W ICM profiles floating around on the web, one is included with Capture 1 RAW processing software (even the demo). You can embed the profile (mine is called C1_BW9.ICM) into your files using PM's Embed ICC Profile command in Tools menu. Now, when you turn colour management on and off (C key), the files will turn from B&W to Colour. The files insides are not changed, just their header with colour profile (meaning you can afterwards again embed the sRGB or AdobeRGB original profile and they are as they were before).
If you do Save As/FTP/... and tick the option Conert to sRGB, they will be permanently converted to B&W!
Best works with sRGB profiled files though (because they look normal on your screen with C off).
To Kirk, actually if you implement this feature (which I think would be useful as well), this could be the way to go - the code for colour management is already there, you would have to add just few lines into the interface code ;-) (for example, toggle with B key?) - like temprorarily assign B&W ICC on/off. You would just have to get somebody create a B&W ICC profile to include with it, or find a free one. I am no expert in CM so I don't know if B&W profile is simply few numbers or if it is more difficult.
Frantisek