I was fairly certain I had done it before. I did some Googling to sanity-check myself and found, "UML can be compiled to run within UML" [1]. I don't seem to recall any unusual compiling options to make it work, but then that was a long time ago, as was that document, I suspect (although it has no dates). Maybe it's broken...

[1] http://user-mode-linux.sourceforge.net/old/UserModeLinux-HOW...