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
