I've fitted a Streetpilot 2610 using the standard BMW mount for the HP2. I'm not sure but I would assume the fixing bolts of the Garmin cradles would be a standard type across the range?
Few weeks back I managed to get a look at Tallgirl's HP2 and (if memory is working ) settled on a u-bolt setup. I did look around for 'nooks and crannies', but do not recall any of the 1" ball on post or thread jobbies were an option, the rail for the screen and clock was a possible.