I updated this evening and finally got a build to complete with the
following mk.conf:

USE_NEW_TOOLCHAIN=yes
USETOOLS=yes
TOOLDIR=/usr/tooldir
RELEASEDIR=/usr/release
DESTDIR=/usr/destdir
MKTOOLS=yes 
BSDOBJDIR=/usr/obj
MKOBJ=yes
MKOBJDIRS=yes

Sadly, the snapshot build died:

..
making base.tgz
making comp.tgz
pax: Unable to access ./usr/share/man/cat9/vn_marktext.0 <No such file or
directory>
pax: Unable to access ./usr/share/man/man9/vn_marktext.9 <No such file or
directory>
*** Error code 1

Stop.
make: stopped in /usr/src/distrib/sets
*** Error code 1