I've been doing some testing of PS2 -> x360 converters and the one ultimarc is using
looks like the one that xArcade and Xtokki are selling. This is a top quality converter BTW. I'm chiming in here because it doesn't seem like you are getting much info. I don't know what kind of adapters Andy is using and might be completely wrong.
The xArcade site says the following combo can activate the guide menu:
There is limited support for the Guide button by holding the Back button and pressing X+Start (all 3 buttons simultaneously), but you may need to do this more than once (thanks to Cory Cacioppo for finding this workaround).
http://www.xgaming.com/support/questions/43/Xbox+360%E2%84%A2+Adapter+Button+Layout+and+InstructionsI know that is an unlikely odd combo. See if you can get this combo to work. If it does, check your iPac mapping to make sure you aren't accidentally making it too easy to hit this combo. This isn't likely but it is
something to check.
Something else I have noticed with some adapters is that they activate the guide when the PS2 devices is first plugged in. If you plug in the converter first then plug the ultimarc converter into it, does it activate the guide? If so, there might be a lose connection you are dropping during intensive gameplay.
I have to say that it sucks that Microsoft added security for controllers that is forcing smart folks like Andy to use converter cables instead of supporting the device directly.