Steve, one thing to be aware of with Mark's board is that it doesn't include the debug connector. The purpose of this is to allow access to the integrated debugger that's included in most of the Co Processors. The debugger is there to make software development on the Co Processor easier. It's very similar to the debugger in B-Em, and allows breakpoints, watch points, single stepping, disassembly, memory dumping, memory editing. You use it via a terminal connected to the Pi's serial port. It's PiTubeDirect's secret feature - mostly secret because I haven't managed to find time to document it yet. But it's very powerful.
Anyway, if you did want to use this, you could easily solder a 3-pin right-angle 0.1" header to pins 8 (Gnd), 10 (TxD) and 12 (RxD) of the Pi Header.
I have let Mark know about all this, but I guess his target market is Tube Elite players rather than software developers.