It's not only Cheapo, but (afaik) also some versions of the actual game where that won't work, so it should count as a glitch, not a backroute. (How do you get 100% though? What about the left trapdoor?)
The 100% solution is tricky. A0;I did it by making the first lemming out of the left entrance dig as soon as he hit the ground. A0;The next lemming bashed through the pillar to keep from turning around, and the digger bashed as soon as the lemmings could not walk out of the hole (7 or 8 swipes, I think). A0;This basher eventually hit the steel on the far right. A0;Fortunately, the basher's last swipe left a piece of ground that was just high enough for another lemming to finish the bash through the top of the steel.
Note: A0;I got this to work in the DOS version. A0;Supposedly, Steaver also found this same glitch, according to this thread:
http://eng-forum.lemmingswelt.de/cgi-bin/yabb/YaBB.cgi?board=news;action=display;num=1094183189;start=15Edit: I just realized that I forgot something in this solution: The third lemming out of the left trapdoor bashed while inside the digger's hole, and then the digger bashed in order to stop digging. By simply making the digger bash, he is a couple of pixels too low for the solution to work through the steel.
To summarize:
1. The first left lemming digs.
2. The second left lemming bashes through the pillar.
3. The third left lemming bashes while inside the digger's hole.
4. The digger bashes when the lemmings can't walk out the left side of his hole.
5. One of the bashers goes all of the way to the steel and leaves a "step" when he stops. Another lemming bashes on this step and makes it through the steel.