Thank you Mazer, will surely look into it. Further search has led me to James Loudspeakers. Their ON-wall models seem very interesting due to their sealed design and the dimensions of certain models are near perfect for my needs. Perhaps I should start looking at On-walls to remove any concerns about the air volume behind In-wall speakers or something which are sealed.
I see a problem with your plan. A wood wall will not have anything like the damping factor of sheet rock. So I would expect a lot of coloration from a large plywood wall surface. I definitely think the speakers need an enclosure. However without knowing the T/S parameters of the drivers, it is impossible to know the optimal enclosure.
I just did an in wall design, MTM sealed left and right, sealed three way center, and in wall TL sub. Certainly there were some problems from reflection from the wall and TV, that reinforced part of the bass spectrum and required a lower crossover than calculated. However the project was great success, and has excellent WAF factor.
Talking of WAF my wife and architect/interior designer daughter had discussions on the grill color and trim. I was in a mind to match the wall and trim to the wall color, but my daughter especially said the fabric needed to match the TV and the speaker trim match the TV bezel. The white cabinet trim to match the cabinet and be white. I think they were correct, so that is what was done.
Here is the FR of the right speaker at 1 meter on axis. The calculated F3 is 83 Hz, but you can see the wall effect starting just below 200 Hz, and there is a rise in response below 100 Hz from the wall. This actually lowers the F3 by an octave to 45 Hz. That made the optimum point to cross to the sub 40 Hz. Higher and then there was huge wall reinforcement of the in wall sub, with the driver being right below the center speaker, the TL port being just the left of the power amp cabinet close to the floor.
Anyhow it was a fun project and I learned a lot.