No personal experience, but SIMH supports the PDP-7,
and there is a "macro7" cross-assembler at:
https://github.com/simh/simtools/tree/master/crossassemblers/macro7
Tho I did debug hand entered versions of DDT and MACRO for the PDP-1,
so I've experienced the particular kind of pain involved in this sort
of project... I even adapted the SIMH macro1 cross-assembler to be a
more faithful emulation of PDP-1 MACRO.
See
http://www.ultimate.com/phil/xy/
Phil