UV maps aren't supported for custom blocks, this is because Asset Forge uses world based UV mapping to make sure textures are seamless and don't get stretched across blocks. So unfortunately this isn't supported.
Interesting -- so if I understand right -- I could bring in a block with a material, and no UVs, and Asset Forge will just tile that material on to my block in world space automatically? -- I could see that as being really handy...
It would be nice though if I could disable the behavior on certain blocks (like character placeholders as seen above) -- maybe this is a feature request then and not a bug in that case. -- I suppose for now I can just map solid colors or something on to my character placeholder.
(I'm really hoping to use this tool as a level editor, and I want to make sure I operate in the correct scale, so for now colors is fine, but UVs on certain objects would be nice).