IF
The top core would be merely a surgery to remove the foam and replace with solid.
That's a pretty good curve to the top and a structure incorporating the side strut supports with the cross-beam into a single piece could be made very strong. And essentially non-deflecting. Too bad we don't have a resident engineer on the board! This may be a perfect project for carbon fiber. Or titanium (just kidding).
If one could take a bit of intrusion into the interior maybe laying in a tubular sectioned curve by cutting the top of the cabin completely out (and reconnecting it back up would get the bridge. Or putting 1/2 of the section outside and the other half in. It would create an deeper bump which might not be the clean look you want.
One could 'play' with the outside beam, designing in rigidity, by incorporating a tabernacle. Like a partial truss. I feel that interior posts and pipe that are built off the furniture are secondary. True support would be continuous (contiguous) to some load spreading floor/rib/contraption actually on the keel and hull.
Just noticed (working on the port V-berth tank in 338) that I doubled up on the tank end that is on the remnant of the compression bulkhead. End supports for the compression beam end on the V-berth as Pearson had it - but instead of depending on screws alone coming in thru the plywood, now there is SOME more solid support (1 1/2" plus frp) from the hull under those struts. Again, I feel you have to consider non continuous support as ACTIVE. Meaning: expect movement to occur.
External beam sounds too good to be true. Has it been too easy for builders to steal space from the accomadation to support the mast? Maybe it is time....