Hi Jack,
I was thinking along the same line about a clip/collar on the screw but would have liked a response from Michael on this. The other option I was looking at was to fix a block on the slide and another on the base to act as a stop.
I have built four CNC gantry routers and have had heaps of trouble with false triggering of limit switches. It becomes such a pain that I end up disconnecting them. You might have better luck than me as you have different electronics so I do not want to discourage you from having a go. Given my history on this I would much rather have a mechanical stop so I don't end up searching on the floor for weeks trying to find ball bearings.
You have me thinking about counterweights now and of course there is the enclosure and flood coolant to add to that list.
Good luck with finishing off your machine and I'll keep watching to see how you get on.