For no good reason, I’m thinking on a multi barrel Ruger No.1 or Browning 78 and I’m looking for critical comment/thought.   

My thoughts (sort of)=>
1. Barrel nut probably won’t work due to forearm hanger under barrel.
2. Interesting reference is the H.I.T. Bolt gun system.  The barrel threads in hand tight and is locked in place by a key mechanism that is part of the action.
3. Other interesting reference is the WTO Switchlug system which appears to work by locking the barrel (hand threaded in place) by stretching the threads. (Looks sort of like double nutting to lock the barrel.)
4. Less interesting but used is the system of interrupted threads. The barrel still has to be locked in place. 
5. So, how about a barrel that is hand treaded in (has to achieve proper headspace and extractor slot placement) and locked in place by a body-fit bolt through the forearm hanger?   This, in theory, would kill 2 birds with one stone by applying some pressure to the forearm and locking the barrel in place.   
6. If you’re still reading, the issues I see are:
a. Very limited clearance between the hanger and the barrel.  That impacts how the rifle is fitted with sights.   
b. Fixed sights are not too bad (rear sight mounted to receiver or use a tang sight) but mounting a scope would be a challenge. 
c. Rear scope mount could be attached to the receiver. This is standard on the B78, I think. The front mount could be mounted just past where the forearm hanger ends so that there is no interference.   I am not sure if that would put it too far forward but that could be addressed with a bridge (think picatinny rail but “sexy”). 
d. On the sexy side, a half octagon - half round barrel might look nice and make things a bit simpler. 

Any thoughts/suggestions?   Anyone know a good gunsmith open to such stuff?

I think Savage rifles are put together that way.   BBL screws on and the nut is tightened against the receiver.     That'd be my choice for a switch barrel.   OTOH, my brother bought a Shuttleworth Stevens with switch barrels, and then bought extra actions, so as not to have to trouble with changing barrels.   
CHRIS

I'd probably consider anything but the Ruger if I wanted switch barrels. The Ruger hanger limits not just how it can be done, but also limits how large a diameter the barrels can be.
Of course you could use the Pope takedown system which allows barrels to pull straight forward, so no turning at all when installing, or removing barrels. Just threads on the receiver, and different count threads on the barrel, with a taper to the barrels to hold it tightly.
This is my Pope-Ballard 3 barrel set with Pope's takedown system patented in 1890.

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Your method thru the hanger is what I did with my Miller. I have a thru bolt that my forearm slides onto to free float the forearm and it has a set screw thru it that locks the barrel right in front of the receiver.

Why not do a takedown system like Savage used on the 99's?
Quarter turn and the barrel slides right out.
Or, as mentioned, try the CPA system.

ISS

Current thoughts based on your input =>
* CPA type system
* use “dog point” type Allen head set screw through hanger and into barrel shank. 
* take down set screw to be located under the hammer spring assembly.  So hammer spring has to be removed to swap barrels but the hammer spring assembly would prevent the take-down screw from backing out.   

Need to:
1. Determine if hanger is large enough to accommodate a set screw without being compromised. 
2. Need to figure out how to install sight (scope)
3. Need to find a critical but willing gunsmith.