Close, but no cigar.

I had already tried google and found the code snippet you found,
but that program only does unix domain sockets and doesn't have
anything like the complexity of arguments found on the empeg, so it's
probably completely unrelated.

I just found where userver is called: in /empeg/bin/d it is invoked as:
userver -1 -d ip/tcp=9999 tar xf - -C /
This seems to be part of the install or upgrade procedure.

It looks like a very handy tool to have around for bootstrapping
software development. It means we can squirt arbitrary files at
a completely unmodified empeg.