the primary return of a modern glider is close to nil
Indeed, and since (in the UK anyway) most of the remaining Class E is only there because the gliding fraternity requested it, it seems to me that if there is a safety problem, it should be incumbent on both sides (IFR CAT and VFR gliders) to sit down and sort it out. Some airlines have a commendable risk-minimisation approach by phoning the relevant gliding club before using routes that fly through "their" airspace. But that still leaves heavy reliance on ATC to detect non-transponding gliders. And that's where there's a new problem. Most ATC primary radars nowadays are plot-extracted digital radars that don't display every 'paint', but chunter round a few times before deciding whether the paints they're seeing are moving in a way that's likely to be an aircraft. That means that gliders, which may only paint once every few sweeps, are less likely to show up on those radars than on their analogue ancestors.
I wonder if one practical measure would be to require controllers to inform IFR traffic when it's about to enter Class E airspace? At the moment there's no requirement to do that, so a heavily-tasked pilot in the arrival phase in a busy TMA who is transitioning from Class A to Class E is given no reminder/hint that his/her lookout and crew resource management might need to change. Yes it would require an additional ATC transmission. But isn't that better than the risk of a collision?