Currently if I do:

D("old.com",...
  CNAME("c", "google.com.")
)
D("new.com",...,IMPORT_TRANSFORM("foo.com"))

I will end up with:

CNAME c.old.com.new.com. -> google.com.new.com.

But nowhere else is that record defined in an A record or anything because it is outside the zone.

I'm wondering if the behaviour for CNAMES should first check if the cname is a FQDN outside the old zone, and if so, do nothing to it at all.

That would give us instead:

CNAME c.old.com.new.com. -> google.com.