The /dirtyrestore flag will do this for you - TAC should support it (I'm not TAC so I can't speak with authority, but it works and I know they've used it and QA tests with it).
The primary reason I check for a clean system is to prevent accidental disasters - I had a site that wiped out their production system by accident (they got confused, so they say, between the backup and the restore) so I put hard checks in there to not allow the restore to execute unless the local system was a new install (i.e. fewer than 10 subscribers, DLs etc...).
In all cases the entire local SQL db gets wiped out regardless of what's in there - so you can use the /dirtyrestore flag if you're careful (i.e. don't get confused about what system you're running this on!). The primary thing is that the local system is installed with the same version and is running properly - if that's the case, DiRT will be fine regardless of if there's a bunch of users/handlers populated already or not.
