Over Racing, based in Japan, used to make frames and complete kits. The quality of the frames was outstanding. Not sure if any made it to the UK though.
Red Fox Grinta specialise in Classic Ducati race frames in Belgium, they make TT-type frames and vary them for the DS motors. The website’s basic, they prefer to talk to people. https://www.redfox-grinta.com/
JHP are probably one of Pierobon's biggest customers outside of racing teams, and I very much doubt they wold have earned the reputation they have, with the kind of customer service you describe. If it were just the frame you were after, whilst expensive, theirs would offer you a solid base for a build project.
That would worry me even more actually. They no doubt push back orders from the likes of myself multiple times to put resource where it's needed to keep customers like JHP happy.
Or you could just order it through JHP. I bought a Pierobon subframe from them for my 1098 some years ago. Delivered within two weeks.
Should you lose all reason and decide to build a Ducton, or Norcati, I have a Wideline Featherbed frame, with swingarm, sitting around looking for something to do...
As a 916 and M900 owner, I can confirm that the bottom end/crankcase is essentially the same (externally anyway) and given 916 along with, presumably all Desmoquattro? have significantly larger heads due to the more complex valve gear. Then I'd say a 900/1000DS would fit in anything designed for a Desmoquattro.
If an aftermarket frame is used , what is the procedure for keeping the DVLA happy ? Do you need to get it stamped or engraved with the original number ?
Over the years The rights to build Rob North frames has passed through several people In the early 80's I bought one new to build a Rob North BSA Triple I had a receipt declaring it as new. It didn't have a frame number so I made one up and stamped it myself. I had to take receipts for all the stuff I bought to customs and we had a discussion about the donor bike (for the engine) and they decided that could be scrap value. I had to pay them 10% tax on the total. I then get it MOT'd and applied for V5 and road tax It was registered as new
Thats just bike porn .... Having cancelled my V4R order i am now trying to price up an X85R build .... hmmmmmmm