Hi Vee-tail
To take your points in turn:
If we continue our thought experiment, and replace the gliders with PA28s. Let's say we're operating at typical glider levels - between 1500 ft and 5000 ft.
1. The semi-circular rule. Which is safer - to put all that traffic at a few levels (8 in my example - every 500 ft, even assuming they apply below 3000 ft), or to have them at all levels? I think I'd argue the latter - you're much safer with people climbing and descending. If the cross section of a couple of aircraft is say 20 feet (ie they have to be within 20 ft of height to hit each other), then you have 175 levels available if climbing and descending like gliders, only 8 if flying semi-circular. Much lower risk of collision with the glider like behaviour. (And I'm as guilty as anyone - I have a habit of flying around in power at 'exactly' 2000 ft or whatever - it's the instrument training Much safer to pick a random altitude, eg 3420 ft and stick to that. Or pretend to be a glider and go up and down).
2. Advisories from ATC and squawking for radar: More or less useless - see earlier arguments. It's OK if there are only a few aircraft, but this is a busy day, remember. Too busy for a traffic service.
3. Radio: As (2). Only tells you to keep a good look out. You know that anyway.
I wasn't aware that Unicom was for broadcasting position reports in the Open IFR. I thought it was for airfields without a dedicated frequency. That might just be my misunderstanding though. But I'm not surprised that you don't get a response on the gliding frequencies - I'm not sure what you're expecting. They are mostly very general frequencies - it's a bit like broadcasting on London Info and expecting people to tell you about what's local to you. Even less useful than a basic service from an ATC unit.
Places to avoid gliders: Above the convection layer; (Mostly) below 1500 feet. (Mostly) not under cumulus. That's where I'd go if I was worried.
Paul