Shouldn't be a problem for the MOT. The front wings and bonnet while obviously a bit stronger than fiber-glass do not contribute to the structural integrity of the load bearing areas near suspension or steering components.
The MOT man will be looking for secure fastening of the flip front, to make sure it can't blow off at speed and cause an accident.
FG is a lot stronger than most folk give it credit for. The majority of a Minors strength at the front, is the chassis rails, inner wings, and tie plates.
That said, I would consider a cross brace between the inner wings a good idea. It will give better front end stability, and if positioned just right, the top can be padded, and act as an anti drumming surface for the flip front, which could reduce crazing of the FB and paint.
One things for sure, the flip front wont rust
