I can't condone the actions described in the first post but I do understand some of why it happens.
Cross country gliding is a high-workload endeavour, especially when the conditions are less than perfect. If you're not used to R/T (it's not part of learning to glide, unlike most power training) and are navigating by 1/2mil in a sailplane of moderate performance, there is precious little capacity left for 'chat' without compromising the ability to stay airborne.
A MATZ doesn't apply to gliders, in the strict sense, although an ATZ does. If you extended the centrelines of all the active runways in the UK out to 10nm and drew the result on a map, it would be a mess, with intersecting lines all over the place. Yes, it is good airmanship and polite to make yourself known to airfields close by but it is not required, if you're not going to transit their airspace. If you're in the open FIR on approach to a particular airfield, you should expect all manner of heavier/lighter than air objects to be rushing/floating about, right up to the boundaries of airspace. Many will be non-radio.
As far as visibility goes, the extremely low frontal area of modern gliders makes them difficult to spot, whatever colour they are. A plus point is that they are rarely flying on a constant track or holding a steady speed and altitude, so there is often some sort of differential motion that the eye can detect, rather than being a 'fly on the windscreen'. I don't think orange stripes on the wings do very much, if anything, as I've often spotted another glider and when it got much closer, observed that it had stripes.
FLARM has had a large take-up in continental Europe and is becoming widely established in the UK. (For those that don't know, it's like TCAS but without the guidance.) It is a useful enhancement to a good lookout, especially in situations where the geometry leaves one or both parties unsighted until quite late.
As far as transponders go, if you got every glider, hang-glider, paraglider, etc. to carry one and use it, within 10 minutes on the first good summer's day NATS would be begging for them to be turned off as thousands of returns swamped the SSR system and TCAS events were triggered all over the place. ADS-B may improve this scenario but it's still debatable. I have a transponder in my glider and use it a fair bit when transiting CAS, getting a Basic Service from an ATC unit or when I'm in IMC.
Also, you have to look at what various groups define as a "near miss". In the LTMA it's three miles and 1,000'; for me in my glider, as long as the other party has seen me and is avoiding, I don't really mind how close they get as long as they don't force me to fly through their wake. I've been in thermals with 50+ other gliders at the same time and as long as there is some discipline it's fine.