I coat everything with polyurethane (spelling!?!) gloss varnish, even the parts that will be covered with groundwork and it seems to do the trick. 

I think you should be fine from what you say. As long as the frame is glued all the way round, the board should hold it in place. I only stain and varnish my balsa wood on the outside and don't do anything where it touches the celluclay.

I have put celuclay onto a narrow piuece of pine shelfboard and despite sealing it, it still bows. You could add the lacquer to be safe, but i think securing it to the base is the most important thing.

The poster board will be glued in the frame.  It is 1/4  inch thick and Frame rises about 1/5 inch above top of board but it is beveled away from the poster board top.  Do you think I need anything or maybe just a lacquer clear coat?

Will you be attaching the frame to the poster board, and how thick is the poster board. If the frame only stands above the board a little you should be fine. I make a frame from balsa wood around a wooden base and never have any issues, but the frame only goes a couple of mm's above the base.

To protect the wood frame from the wet celluclay would it be better to use lacquer clear coat /dull cote OR a brushed urethane coating?  I want the exposed frame to stay uncolored.  Thanks for your ideas,

Leddy

I have no experience with celluclay, so I don't know how wet it is. I usually use drywall plaster, and have not had any problems with it effecting a wood frame. I would emagine that a frame that has been treated with a clear coat, as most seem to be, would offer at least some waterproofing (???). 

 If I were really worried about it, I'd probably wrap the frame with plastic wrap/cling wrap type stuff. I'd continue on, leaving the wrap in place to protect the frame, until everything is done. Then, use a new blade to cut the plastic wrap right where the "ground" meets the frame.