This is a test map showing the Custom UI limitation. If only we could get Blizzard to add the functionality to initiate targeted abilities from triggers, then Custom UI's would be much more feasible.
In the example I am using mouse move tracking to try and create a 3D splat (similar to what you normally get when casting a targeted ability from the command card). In single player testing it works okay, but online it's just not usable due to latency.
In one of my released maps I was previously using inventory items as part of the custom UI since you can click on an item to initiate a target ability, but it was too clunky as the inventory/item system is quite restrictive and not customisable enough.
The other limitation is that you can't hide units per player, however you might just be able to create an actor or model and do this and move it instead of a unit like I did in the example.