Why not use L4?
There is an effort to port the Hurd to L4.
The HURD on L4 web site says the project is over.
I don't know what web site you are talking about, sorry. The Hurd
pages (http://hurd.gnu.org) have no information about Hurd/L4 since it
is still in early development. And these are the official pages for
whatever is concerning the Hurd project.
So is there any hurd/l4 work going on?
Yes. See http://sv.gnu.org/projects/hurd.
Also: Is the gnu/mach kernel with a typical complement of drivers
(I know that's vague, but I can't think of a way around that)
installed small enough to fit into the L1 cache of a garden variety
cpu module?
Doubt it, GNU Mach is around 700Mb gziped. Nor would there be any
gain whatsoever other then by a small constant factor.