I think you could do that with a Why bike, have a look at Cyclorise as I think that is what the owner built up.

I bet if you have a chat with Sonder they will build their Pinion Signal or Broken road with the drive you want.

Nordest.
Mrsstu has a Bardino. It's a well made frame and they were easy to deal with.
https://nordestcycles.com/en/product/lacrau-2-ti-frame

Sonder Signal Ti pinion (not sure if it has the split for belt drive but as each frame will be made to order it's easy to add).
https://flic.kr/p/2jRCinC
They had a demo bike a while ago but not sure what happened to it.

Nordest Lacrau 2 Ti
https://flic.kr/p/2kToUNw
My bike 😉 - built in early 2021, build thread here https://singletrackworld.com/forum/topic/new-bike-build-titanium-pinion-awesomeness/

Any questions, let me know.

Kingdom will do a custom vendetta with pinion/gates if asked.

The Kingdom and Nordest are both fairly slack, hardcore hardtails, the Sonder is a bit more trail orientated.

Most of these ti frames come from the Waltly factory and are done as custom orders.

After a longer wait than expected my 50th present to myself is here and the build underway.

A 3-4 month build queue at point of ordering ended up being 9-10 months until arrived. Pinion delays and other issues. But at least here now.

Curtis Am9 Custom with Pinion belt drive.

https://flic.kr/p/2nF8nRT

https://flic.kr/p/2nFdrFh

https://flic.kr/p/2nCt4rf

https://flic.kr/p/2nCo2KZ

Curtis Am9 Custom with Pinion belt drive.

Ooh, that's rather nice! Pics of the full build, please! 🤤

You could try http://www.spanner.org.uk/

Very pleased with the frame and fork I got from him

Tallpaul will do.

You can take part credit as it was a combination of seeing your build pics and then seeing a Curtis with pinion which led me down this path.

What started as in theory a lightish custom steel frame (normal gearing) has ended up far from it once I went down the gearbox rabbit hole.

Olsen Bikes on the South Downs ticks those boxes 🙂

OMG that curtis...

would be interesting to see how well the belt works in the deep winter sticky gritty crud.

Nicholai will probably build you what you need also https://en.nicolai-bicycles.com/bike/argon-gtb-pinion/ EDIT - not ti though...

would be interesting to see how well the belt works in the deep winter sticky gritty crud.

No issues last winter
https://flic.kr/p/2mWhtkQ

https://www.ebay.co.uk/itm/225037002269?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=5Iez2I_TTmO&sssrc=2349624&ssuid=6Fcb5-HYRwK&var=&widget_ver=artemis&media=COPY

Try Mike Travers of...Travers bikes.

Sonder Broken Road or Signal, depending on how aggressive or not you want the bike to be.

The almost complete photo (cranks sent were wrong ones so slightly delayed things).

https://flic.kr/p/2nGTCn1

And never got a shiny finished photo before just had to go get it dirty. Loving it so far.

https://flic.kr/p/2nGVQPz

Not cheap but good reviews:
https://www.pilotcycles.com/frame-customize/locum-pinion/