Can it be done?

What’s a good frame to use?

Would probably be wanting to run disc brakes

29er frame?

Easy, as long as the top tube/stem combo let you run drop bars without bashing your knees.

Does it have to be a 29er frame? I’m pretty sure I’ve seen a 26″ wheel on one inbred converted to 700c cx. Can anyone confirm/deny this?

you can run 700c on a standard Inbred – would raise the BB up a bit if you had fattish tyres on it though.

Why not just set it up as a 26″ tourer?

Not discs ,but back in the day I used these to run 700c wheels on a Kona Explosif .

26″ Inbred with 700c works fine. Pretty much any hard tail should if its disk equipped. A 700c with a thin tyre on is the same dia (roughly) as a 26″ with a fat tyre.

A 700c rim with a fat tyre is a larger wheel – a 29″

So most hardtails will take a 700c disk hubbed wheel

If by making a crosser / tourer you mean drop-barred then the issue you’ll face with a MTB frame is the TT being too long for drops rather than the wheels.

EDIT if the plan is to swap wheels back and forth so you can use the frame as both and MTB and a road/tourer (which is what I used to do) then try and get matching hubs on both wheel sets (or shim one set to match the other with Syntace shims) so that you can swap over without having to realign callipers

Can be done but top tube will be too long.

If you are buying a new frame, buy the right one.