I've finally made time to head back to the Service Manager forums and I've just spotted this discussion thread:
Rob Ford has greated a great little management pack that adds a task for assigning an incident to an analyst, but uses up to 3 AD groups to filter the list down rather than being presented with all the users in AD as is the annoying OOB experience.
It adds a task:
You can configure the groups under settings in the Administration Wunderbar:
And finally, a pop up window to assign the incident with a filtered list!
And just to make it even better, he's made the source code available too so you can customise it even more.
Brilliant work Rob!!