It seems there’s no way to use Loft with a profile that’s a contiguous union of closed paths. For example: a circle with a diameter line. Selecting the two semicircles lets me use Extrude to create a single body, but Loft disappears. Clicking Loft and then selecting both halves results in an “Invalid loft profile” error (well, yes, it would be, but I’m not trying to loft between them; they’re coplanar). The only solution is to first trim away all interior lines and curves, but sometimes I need those for constraints or other features.
In another thread someone from the Shapr team asked about use cases for construction lines. I think this is a good one. Alternatively, the Loft tool could be improved to support compound profiles.