[9fans] mounting channels in a Plan 9 server?

[andrey mirtchovski <mirtchov@cpsc.ucalgary.ca>]

what's the best way to import a file hierarchy into a 9p server?  for
example, when i'm serving a set of files (A) and want to add to that
set another hierarchy served by a different program (B) on a file
descriptor i have.

trivially it could be done using general plan9 tools by mounting A
somewhere, mounting B in one of the directories served by A and
srvfs-ing the whole thing, but what if I don't want to post a file
descriptor in /srv?  looking at the devsrv code in /sys/src/9/port
doesn't have a solution, since all the 9p messages there return
channels, which doesn't happen in userland...  i also looked at the
QTMOUNT mode bit but there are exactly three examples or it being
used, none of which tell whether it's the thing i need.

so my question is: is there a possibility to say 'here is a file
descriptor/chan you shall use for your next request' when a client
walks a file hierarchy i'm serving?  i.e.  is there a way to mount a
file descriptor in an in-memory server?

andrey