Is target disk mode a clean way to install (Mountain) Lion on a blank mac? Since I don't have an optical disc, and no USB thumb drives are available, but I do have a second mac, target disk mode over FireWire seems like the only way to install Mountain Lion (since this 2010 MBP doesn't have that permanent recovery mode in its firmware, and I was dumb and formatted the whole disk, instead of just the system partition, thus deleting the recovery partition).

But I'm thinking, when you install on a local disk on a MacMini, is some aspect of the installation MacMini-specific? After installing from the MacMini, will the OS be missing some MBP drivers, and include superfluous Mac Mini drivers?
You should be fine installing that way. The OS X installer is pretty flexible, and is designed to be able to install on external drives (which is essentially what a Mac in Target Disk Mode is). With the exception of the system disks included with Macs prior to Lion, which were machine-specific, OS X installs work on any Mac the OS supports. You can even boot a Mac from a different system's drive without issue, as long as it's not a version prior to that Mac's release.