As I type this my 85 300td sunroof pan is in the garage with the POR-15 based repair drying. I had the same exact problem, and decided to just bite the bullet and remove the entire sunroof, pan, and do a complete repair. Luckily, mine only had the rusted pipe problem really bad on one side. The other front side was starting to rust, but had not started to "disappear". The back drains were fine. I think the fronts do most of the draining, and are most vunerable when they clog.

It's not all that bad of a job to get the pan out, but there is this real gooey foam weatherstripping that makes a mess of your entire shop! Sticks to EVERYTHING. I striped all that old gonk off, and will find something better to replace it.

I filled what was left of the rusted pipe with clay, then used POR-15 and fiberglass to construct a new pipe. Of course, all the other rusty areas were treated to POR-15 also.

I think if I were hiring out this job, I would probably use in interior shop as the primary contractor, to remove the headliner and pan, then take the pan to a rust specialist body shop. Then, let the interior shop reinstall the whole thing. I think a regular shop wouldn't have the finesse needed to work with that old headliner.

Good luck.