Yeah, that's a fairly well-known trick, not sure how often it's been featured in custom levels though.
The
challenge board on this forum probably has a number of usages of this trick scattered throughout as well. The latest example of its use can be seen in certain challenge solutions for Beach 7 in Lemmings 2 for PC. Here's a
screenshot of it in action (plus other unrelated things, but you should be able to tell the parts where a basher was interrupted mid-stroke, in that level using jumpers instead of builders). [note: download URL requires log-in to forum to work]
And it looks like from your screenshots that you've gotten the hang of level editing with LemEdit and CustLemm.
Hope to see some fun and interesting levels soon!