Hi Rich, so if I understand you correctly you can get the result you're looking for in Moi using the "Construction lines" function.

There's a bunch of info on it here:
https://moi3d.com/4.0/docs/moi_command_reference11.htm#constructionlines

You activate a construction line by holding the mouse button down over a point and keeping the button pressed and dragging, instead of just a press and release.

When you make a construction line a little arrow tag will appear and if you click and hold down on it a menu will pop up with some additional controls, one of which is to project the next point onto the construction line which will do the type of thing that you're describing.

So for example here I'm going to place the first point of a line at the corner of a rectangle but at the same height as some other geometry:





Instead of clicking on the point, keep the mouse button held down and drag away from it, that will make a construction line there:



After dragging some distance away and releasing the button, a little tag will appear:



Press and hold on the tag and you'll get this menu:



Choose "Project next pt" and then you can snap to some other point and inherit its height like you want:



- Michael