For mine the smart play would be invite a guy into training for a couple of weeks to see how he trains it would let guys know they are being noticed and the hard work is paying off.
It would cost some kit and that is about it so would be cheap.
I am fairly sure they already hold these sessions as part of a pre pre season type thing before and just after Christmas. Kind of like a wider training squad before the super and club seasons kick-off.
Part of the problem with getting blokes in during the season for sessions is most of the club players are strictly amateur, as in they've got jobs and families outside of rugby. If they're already training 2-3 times a week, plus gym sessions for their club, finding time to get some worthwhile sessions at the Reds would be quite taxing.
To your point though, each club in QPR has a bye week. Maybe the Reds could look at getting a half dozen blokes from each club to attend Reds training during their clubs bye weeks, that way they aren't double dipping on sessions or missing the leadup to games with their club. As you say, chuck them some kit and a pat on the back and say you're doing well. Doesn't take much.
The reality is, with overseas money coming thick and fast, the Super Rugby clubs are going to lose more of that middle tier of already developed professional player. Getting a decent look at 30 odd club players across the course of a season in their own training environment will be crucial to help them fill the gaps each year.