It might bond temporarily, but it will not bond long term.Tipjar's not going to have any floatation, not even sealed bulkheads? Seems like a really bad idea to me, especially on a small boat? I'd rather have to rebuild the boat in 20 years than sink after a mishap that foam would have saved me from.
On the PT stuff you've seen, any evidence of resin not bonding to it well?
Don't use it as as a stringer and attempt to glass it to the hull.