There doesn't seem to be an equivilent of the Maximimum permitted mileage at BMI - just a 'how sensible is the routing and is it a permittable one according to the standard *A timetable. If your routing appears on staralliance.com when searching for the start/end airports you won't have a problem. Otherwise the standard advice is to state your route sector by sector and you should be fine.

However BMI does frown on backtracking, but I think in this case - considering the stopover, you'll probably be fine.