Thanks for the poke on this. I had looked briefly and become frustrated that many of the instructions I found assumed you were a kernel dev and started with compiling everything from source— the Debian-supplied UML binaries seem to work well for my needs though, and do indeed allow doing basic stuff like mounting a disk image so you can run install-grub on it.
Hmm, building UML from source is really easy. Here [0] is my process for doing it, as a Makefile. The actual compile step is just one line (line 32):
$(MAKE) -C linux-$(KERNEL_VERSION) ARCH=um linux
The rest of it sets up the configuration how I want and compiles other dependencies (like slirp) or is for maintenance, like cleaning up, or downloading.
This is a rather old version -- newer versions check the checksum and use my HashCache system.
Minor quip though since ptrace might even be slower than vm exits; your core point stands.