Yikes! Good thing I didn't know how comlicated all this fixed/free stuff was or I'd have never tried it;-)
I'm riding an ancient Atala right now. 120 rear spacing, foward-opening angled/horizontal dropouts with no derailleur hanger, and cheap Surly fixed/fixed hubs. (Surly does, BTW, offer free/free hubs).
I use a 19 tooth White Industries freewheel on one side and an NOS 24 tooth "A&C" (Belgian made) freewheel on the other. No problems, and yes, I do "flip-flop" the wheel in the middle of a ride, at the bottom of the mountain. (The angled dropouts permit this without requiring brake shoe adjustment. Why the modern frames intended for flip-flop hubs use horizontal dropouts I do not know.)
I'm a bit worried about the limited thread engagement, and the amount of freewheel threads that are left unsupported, but I've had no problems so far, and have done some "kilo starts" on the bike.
As far as lockrings go, I haven't used one in twenty years of track and fixed gear road riding. The only way I have been able to loosen a properly tightened but "unlocked" fixed gear on the bike is by hopping the rear wheel while back pedalling, and being as I was never a match sprinter, I avoid the issue by just not doing that.
There was a time when the whole racing community put on a fixed gear for the winter; just spun it right on to the same road hub used for the regular freewheel, and I don't ever recall seeing any problems with it. (Roadies getting launched into the air after a whole season of freewheel riding excepted, of course;-)
Noel Hoffmann
Huntington Beach