The first thing to determine is whether you can /record/ back onto the camcorder. Only a small proportion of camcorders will do this and, in any event, the signal would have to be in DV format. So I suspect that's going to be a non-starter for you.
However: "... I have used my Canon DV Camcorder many times in pass thru mode but this has always been to transfer VHS footage to the PC ...
I'm not familiar with this arrangement but can you not do the same sort of thing with the portable DV player? That is, connect the DVD player to the camcorder in exactly the same way you (presumably) connected a VCR?
Thence from the camcorder (pass through mode?) to the PC.
Forgive me if this is rubbish; as I say, I'm not familiar with being able to use a camcorder for "pass through". But HTH.