Is it possible to build a kernel when /usr/src is mounted read-only?
   
   Is there a mechanism to make a kernel in /usr/obj?


i'm not sure if this really answers you question (ie, RO /usr/src), but i
use this to build kernels on machines that don't have /sys locally, but i'd
rather build on local disks:

cd /sys/arch/pmax/conf; config -b /orb/p/_aubrey-ii_ -s /sys _aubrey-ii_

which will build my pmax kernel in /orb/p/_aubrey-ii_, while looking for
sources in /sys.  someone added the -b/-s switches about a year or so ago.


.mrg.