Not sure that the question was silly. Some bikes have welded on flanges for bash guards. You're looking to add a guard to a bike without that flange. So you propose using an adapter flange that fastens to the BB via one of the BB nuts. I'd think that the force transmitted in each case might be different. The three cases:
1) No guard, force transmitted via chainring to spindle to bearings to BB in frame.
2) Built-in bash guard flange. Force transmitted through guard to flange.
3) Adapter flange for bash guard. Force transmitted through guard to adapter to BB assembly and nut to BB threads.
I think (2) is best, but if you don't have the flange, (3) probably works. (3) might avoid chainring or bearing damage if you "log" your chainring.