Can a 5900 series S&W be reasonably converted from DA/SA to single action only? Seems that would make a pretty nice Smith & Wesson Hi-Power.

Yes it can. S&W used to do the conversions in their custom shop a good number of years ago, but I don't have any idea if they still do. I do not know what they were doing, but as I recall, the thumb safety functioned in a manner opposite of a 1911, in that upon presentation you flipped it up to take it off safe, instead of down.

MS

Thanks. Supports the little bit I know. I’ve read one report of this surgery being done and a sample of 1 doesn’t mean much. Can’t even remember where I read of it but it was done by SnW and labeled with some sort of match designation. And the hammer drop was turned into an upside down safety as you say. Wouldn’t be my first choice but not deal breaker.

Yep. The safeties of 39’s and 59’s worked opposite a 1911.
If the hammer is cocked and the safety is put in the down/safe position, it will drop and the firing pin will be blocked as well. The way I think of it is, if the safety is pointing in the same direction as the barrel it’s ready to go.

I prefer S&W safeties pointing in line with the barrels when ready to fire.
Kinda like 1/4 turn valves. In line with piping the valve is open. Perpendicular to the line and it’s open. Easy to tell even several feet away.