Generally speaking wood glue does not like to stick to any kind of finish.
There are those here that have found some glues that they have had success with as far as using with finish. But, as far as I'm concerned the only way to do what you are wanting to do is keep certain areas free of finish by sanding or taping over small places before finishing the back board.
If you can keep a wood to wood contact patch between the two pieces and just use dabs of glue in those areas, they will stick like glue!
When you fill the pores of the wood with a finish, there is no place for the glue to adhere to because the pores are what the glue uses also.
It is just one of those things that you have to plan ahead for.
Rog
