I’ve tried this with a 700c tub on a 29er rim.
I’d say it works in principal and it’s close to working in reality!
I found some cheap tubs from PX that had the threaded valve – That bit is essential but yes, they seem hard to come by – most seem to not have the thread on.
Getting them on the rim at the same time as the outer tyre is a struggle but possible. I used a Hans Damphf evo 2.35 which are pretty baggy themselves. I wouldn’t want to try it with a tighter tyre. Rims are Mavic XM719s with an extra hole drilled for the tub valve and regular tubeless tape fitted. Getting air past the tub to the main tyre isn’t a problem with a track pump and some heft – I think it’s a DT valve but I can’t see it making much difference.
So doing all that, I can get everything pumped up to the point of bouncing the wheel round the garage and feeling that warm sense of achievement. Close inspection though concludes that I can’t get the tub valve sealed onto the rim so I’m slowly losing air from the outer tyre. I’ve tried bonding o-rings onto the valve at the base, building up a ‘bung’ at the base with silicon sealant, bodging a lockring and o-ring either side of the rim…..nothing seems to solve it.
Like I said, feels really close yet, so far!