Downeast, now you've got my attention! First, because unlike that lucky California dog 510, I'll bet Downeasters maintain a working relationship with condensation. Second, because anyone who regards their time working on a boat as beyond value (some would say "worthless") is a kindred soul, and third because it sounds like you know about something I need. What is "a sprayable 2-part marine foam kit"? Is it an improvement over foam-in-a-can or "foam-in your-hair" which is usually more accurate?
Your proposal for the cabin ceiling sounds beautiful although I hate painting mahogany because its so pretty. However, I understand the need for a brighter interior. What about tranverse varnished mahogany 1" trim strips every 3' or so to break up the strong longitudinal lines you would introduce? I saw a similar strip treatment for the 1/4 berth and vee-berth using varnished Western red cedar on the sides and top and it was gorgeous. Only two other questions which I think can be solved. What will you see between the 1/4" gaps and what about the strongly curved vinyl covered sections in the aft corners of the cabin trunk? Any thoughts on what to do about the vinyl covered partial bulkhead holding the port chainplates?
Here is a link to one of the available foam kits. If you decide to do this please take safety precautions and wear a hooded disposable Tyvec coverall or you will have foam in your hair.
Mask everything close by, carefully. Preparation is very important. Rehearse the paths you will follow. The kits come with a "gun" that mixes the two parts in a disposable, removable nozzle. The right kit will include several tips so you can start and stop several times as you use up the product. One tip for the V birth area, one for the 1/4 birth and perhaps two stages in the main cabin....Use a slow foam for maximum depth precision.
I will paint the finished foam surface white. It hardens to a density that you can barely put a dent in with your finger so under and between the wood strips it will look fine. I am thinking I will epoxy short sections of pvc pipe over the fasteners, mask their ends and push a dry (removable) foam plug into each one before fastening the wood strips.
Painting the mahogany? I get it. I will "wash it" with a thinned 2 part epoxy. I don't need any more varnish. The mahogany is available and an appropriate wood. I would not want a dark ceiling in this little space. A clear urethane on the mahogany would be too dark. The transverse trim strips might work. I need to think about that. I like it.
The partial bulkhead gets faired and painted white. The curved aft corners will probably get epoxied and painted white but I will figure out how that gets done when the time comes. It is a fun detail.
Handi Foam SR Two-Component Poured-In-Place Foam