I previously installed a four gang box in a house. The problem with that box was it ended up being that it was so long that half of the box will be in the backsplash of the kitchen counter and half not in the backsplash.
I was thinking of covering two gangs of the box with possibly a piece of metal covering that would go on the device box (than drywalled over). I will replace it with another 2 gang box above or below it (therefore all four 3 way switches will have a home). I really wish they just had three way rocker switches. But they actually only have rockers switches that are a single pole and three way switch in one. No double 3 way rocker switches...
I cannot ungang the box since it's a solid 4 gang device box and I do not want to slide it out to replace with a 2 gang because it took a lot of fastening on both sides of the box, and the wall is finished and painted! (Heck maybe I will take it out).
So I believe from a code standpoint that should be fine since the box is still accessible (from the other two gangs...nothing will be in the end two that I want to cover). So do they have a part for this. I cannot think of a name of one. I just wanna be ready for tomorrow.