I had given this some thought some years ago. Let's go over what you said you need...
I think this could be accomplished, no problem. You could either use Previous or qemu (running the intel version of OPENSTEP).
2) Communication via DPS...
You don't need XDPS for this. The XDPS backend for GNUstep was an attempt to use the Display Postscript that was going to be integrated into X11, but this never happened because the people on the project determined that postscript was better done as a client/app side thing rather than in the X server itself. Here is the discussion...
One place you can get information is from the following website, he has done a fair bit of hacking on PS and DPS...
Of particular interest is project Akira. It may be possible to connect to the DPS server over a port and communicate with it, but no one has done that and I am not sure the remote capability is present either in the DGS/DPS backend that X was planning.
3) Pasteboard bridging...
The pasteboard on the NeXT likely uses the typedstream style of encoding that was common on the NeXT machine. This encoding has never been reverse engineered... one possible resource for this is a python program I found recently that discovers, in part, the structure of typedstream archives...
The tool doesn't get the data it just breaks down what data is there which could be instrumental in reverse engineering it.
CONCLUSION...
As you can see there is a fair amount of work here. This is all of the information I have discovered over the years on this. If I find anything else or dig up any other information that might be useful I will let you know.
Yours, GC