Here's an article on re-bushing the SU's that I found in my e-library.  Credited to Delaware Valley Triumphs club member, Tony Rhodes (www.dvtr.org  -- based in Philadelphia) .  It provides some insights into the symptoms, the cause, and one of the cures (re-bushing).  In essence, this approach assumes that the majority of the wear problem occurs in the steel bushing ('ovaled'), not the ends of the chrome-plated brass shaft.  That seems counterintuitive, but I miked a set of worn shafts and found very little difference in the diameters of the unchromed ends vs those of the chrome-intact sections.  Maybe someone else can verify by doing some measurements on used throttle shafts that they have on hand.

SU Carbs - Rebuilding - Bushing Reaming.pdf

11 hours ago, Captain Obvious said:

You're not talking about cutting internal O-ring grooves into the (bushing that's pressed into the) carb body, are you? If so, call me and I'll do my best to talk you out of that. 

If that's not what you meant... I've seen a couple simple seal versions. First is on the round tops... The outboard ends can be capped off completely. Only the inboard ends need some sort of rotational seal.

And someone many moons ago did a simple "crushed O-ring" compressed up against the outside of the carb body. (@Blue @240260280 maybe)

 No o-ring grooves. I'm much more primitive than that. The crushed o-ring and cap were what I imagined. Or maybe I didn't dream it up it and I don't remember the source.

I got lucky and without a huge amount of digging, I found the thread I was thinking about. It was Blue.

https://www.classiczcars.com/forums/topic/40433-su-mod-sealing-worn-throttle-bushings/

This the best instructions that I found but it doesn't address the throttle shaft itself being worn.

 

https://windyridgefarm.us/documents/DIY_Guide_to_Re-bushing_SU_Carburetors.pdf