MeshY like TerrainY?

I plan on moving npc entities around a "floor" mesh, this mesh probably will end up having elevation changes. Is there a MeshY similar to Blitz terrain's TerrainY?

Nope.

Best way would be to do a linepick straight down from your character position.

Doing linepicks like that would kill my engine... It works fine if it's only a few of them... but I have at least thirty npcs running around.

Only other way is to try using sliding collisions but I don't know how well that's going to work. Might be instances where an object can drop through the floor...

You could try to calculate the position under the npc, using vertexx(), vertexy() and vertexz(). For that, find the 3 closest vertices around the npc and calculate the height using these variables.

You could perform a step when the game loads, or outside of the game and store it as a data file, where the program performs a bunch of linepicks at regular intervals to create a grid array of heights, like a heightmap. Then you could interpolate between grid points to calculate the height of the mesh at any point. As long as you use a fine enough resolution of your grid it would work, and should be almost as fast as terrainy - which is really just a calculation based on the heightmap in a similar fashion.