I'm left wondering if it's 100% necessary to go completely to the wall if you can't find the ideal trencher for this situation.  Are you going to be doing a lot of standing or working right next to the wall?  The only effeciency lost would be due to the frost creeping under the area outside the trenched barrier.  Just be sure not to put the thermostat over this area, or if you do -- keep in mind this area next to the outside perimeter might be cooler and the thermostat will have to be adjusted accordingly.

Maybe you could mitigate the lost efficiency in that area next to the wall with 4" of horizontal foam?  But you'll still have the potential for rodent or varmit damage to the insulation unless you can protect the bottom side of it.

As the previous poster stated, use what you have.