Basically, I want to treat "pkgsrc" as a platform, like RedHat, Gentoo,
  Debian, for which the software already provides startup scripts.

  The patch would add a line

    If this is built with pkgsrc, install rc.d script for pkgsrc

  in front of a list of clauses like this:

    If this is RedHat, install initd-script.redhat
    If this is Gentoo, install initd-script.gentoo
    ...

  Of course I can add a patch to pkgsrc that completely removes those
  other lines and will have to be kept forever in the package because
  the upstream authors will (understandably) not like such a patch :-).

  Isn't it one of our goals to have as many patches as possible fed back
  and removed from our repository?

I have faced this problem on a number of occasions, and have addressed
it not by special-casing pkgsrc itself, but by adding configure
arguments e.g. --enable-exampledir to put rc.d scripts in the right
place.  So I'd go for that; the configure arguments are more likely to
be usable by others for other purposes.

But, I see your point and I think it would be reasonable to define (in
doc/pkgsrc.txt) an environment variable that will be set during build
and install phases to allow packages to act in pkgsrc-friendly ways.

-- 
    Greg Troxel <gdt@ir.bbn.com>