The concrete base I wish to construct is shaped (in cross section, both
ways) like a staple. As a result it will be very difficult to lay a
DPM. Would it be ok to partly lay the concrete to dramatically reduce
the arms, them lay the DPC followed by the rest of the concrete? The
level of the concrete at this point would still be below ground level.

I can't think of any reason why it would be a problem to do that.