High Efficiency Wall Mount Speakers and 300B

How about the 47 Labs speaker. It is a single driver config and is quite small. I don't know if they are ported though?
These are based on EJ Jordan divers. If you are really motivated, buy a pair of Jordan drivers and build their own box. These like very small parameters..... I believe that a pair of the drivers are less than $200. Do a google search.

Keep the 300B, forget the LS3/5A as T notes. Again as he suggests, a single-driver would be a good idea. Maybe something like a cornu is the ticket. Apparently it can be hung against the wall! Usefully, the waf should be better...:)
Caveat: never heard this design.

Benny from diyparadiso (.com), a knowledgeable diyer/ component dealer has, and offers some comments on his site. FWIW!

Carl, LS35A are not really a good option, because they need to be well out into the room for proper sound. For a similar sized speaker that is intended to be placed against the wall, you might consider Linn Kans. They are not high-efficiency either(I think about 88db), but at least will be placed right.

You should really have an efficiency of at least 92db for the 300B.

If you wanted to do some in-wall work, you could make a pair of single-driver Voigt Pipes built into the areas between 2 studs in your wall, and place the vents near the baseboard. That would require some wall tear-out and construction.

It's going to be hard to properly satisfy the requirements you have, without some serious drawbacks.