You might want to write to a few gliding clubs in the Netherlands directly to solicit their input. In the Netherlands, mode-S is now mandatory for all powered flying above 1200 feet, and the lower limit for unpowered flying has been steadily pushed down.
As a result, a significant percentage of the fleet now has now had both FLARM and mode-S for about a year. That's some considerable user experience which you might not want to ignore.
You can probably get a full list of glider clubs through the KNVvL.
Edited: I realised that you might not speak Dutch. I navigated the site and here's where you can find a full list of all gliding clubs and their websites.
Zweefportaal - Weblinks: Zweefvliegclubs in Nederland