Ah... a LeopardAssist question, I may be able to field this one since I develop it. Well I did up until last week or so.
Very simply LeopardAssist is problematic on QuickSilvers, they don't always react well to having their firmware tweaked. You may be able to get away with just simply selecting "None, I will hold down a key" in the configuration dialog when it comes to selecting a boot drive. Proceed as normal and restart when prompted.
Then just hold down Option on boot to bring up the boot manager, from there you should be able to select the installer drive.
We're running through this over on the LeopardAssist forum at the moment.
http://www.mac.profusehost.net/forum/index.php?topic=50.0
Unfortunately the only test system I had for LeopardAssist was the Sawtooth G4. Version 2.0 which was in production had a new way of communicating with the NVRAM which was almost foolproof, and would have corrected these issues (most likely). Unfortunately the Sawtooth G4 used to develop it came to an abrupt end earlier this week and therefore the app can no longer be tested- in turn the app may officially cease development some time next week.