As far as the rules go this is pretty simple. There is no beam of the sort you describe in the original plans or the "building rules and restrictions", and it is not specifically de-restricted in rule 19, therfore it is not allowed.

The Mosquito is a restricted class rather than a development (or box-rule) class. That means the rules say what you can do, and anything else is out.

Anyway that's not to say anyone is going to be too bothered if you put a bar across the chainplates on your boat to take the bridle loads. But if you ever took the boat to a titles you may be asked to remove it for the event.

The question is whether there is any point to doing this. Mosquitoes are designed and built to be sailed the way they are.

If your hulls are showing signs of collapse on the inside face near the front beam (or in the deck) then you are heading for a major failure anyway - even if you do add a bridle-strut. In this case the hulls really belong at the local tip. You would be taking a risk using them, and it's no fun breaking stuff and hoping to be rescued. There are plenty more cheap ply Mosquito hulls around.

On the other hand if your hulls have been stored dry and the timber sounds hard when you tap it and looks even with no bumps, splits or ripples, then don't fiddle with it, just go sailing and enjoy it.