Assuming we don’t want grayed-out squares to have additional height, this seems like a good idea. Earlier I was trying to do it with actors, but it can instead be done in the board’s PS. I haven’t fully digested your answer, but I think a good idea might be…
The PS can take as input the board texture plus a 1-bit 32x32 texture (really 26x19) that tracks which of the 26x19 squares are grayed-out. In the PS, we need to do a reverse modelViewProj (like we do in shadow mapping) to get from screen space to model space (of the board). Then we do two texture lookups. The normal one to the big 2048x1642 board texture, plus a second texture lookup to the 32x32 texture. If the 32x32 texel lookup is flagged true for gray-out, then we multiply the color we got from 2048x1642’s by the grayed-out color scaling, else we just use the texel color from the 2048x1642 texture as is.
That’s how I’d do it in OpenGL… But I’m not sure about UE4. I’ll have to think about that and also digest @HungryDoodles answer.
Update: One thing I notice in your answer is “big enough to contain all other textures as tiles there”. I only need to gray-out the board texture. The other objects don’t need to be grayed out. Sorry about the confusion.