I have had this idea for about a year but never tried it out for real until now. I have written about the bridge plate I do in spruce before on this forum and it's working great. But one thing didn't seem right. Between the ball end of the string (resting on top of a thin hardwood reinforcement around the string pin hole) and the underside of the hardwood bridge there was a whopping 7 mm of soft spruce. It was like having the ball end resting on a spring.
What I just did was to drill 8 mm holes through the spruce top (3 mm) and the spruce bridge (another 4 mm) and fill the hole with a stiff piece of a round birch dovel. I did this as an afterthought, next time I will drill the holes and glue the dovels before I glue the bridge. You only need to support the upper part where the ball end rests, the goal is to have only hardwood between the ball end and the bridge. May not be a good idea to have all the force from the ball end pressing on the birch dovel, it might make the bridge come loose...
I was amazed by the volume when I got the strings on the old European parlor. By far the loudest parlor guitar I have ever restored! The tone is clearer and the attack snappier too. Especially the two unwound strings sounded better.
This is a success and I'm sure any guitar with a standard maple bridge plate and a 3 mm spruce top will benefit even if my setup is unusual with that much spruce between the ball end and the underside of the bridge :-)
Been doing some more experimenting. I remembered my own opinion that nothing beats spruce in the guitar top. I found the sound from the birch plugs to aggressive for my ears and tried spruce plugs. After all the guitars are small parlor ones with no need for extra trebles. The spruce end grain is harder and hard enough to make a difference. Now I have more volume, more sustain, a clear tone and better separation without the harshness from the birch.
When I compare with a guitar made the same way but without these plugs is sounds really muddy, With the plugs single string playing within a chord really stands out. The one with spruce plugs is probably the best sounding one I've ever restored!