Have heard rumours that in the Netherlands they've had problems already and have asked pilots to switch off their Mode-S when near controlled airspace due to the volume of spurious TCAS alerts.
Not a rumour but an established fact for quite a few years. Prominently in the AIP - I think in ENR 1.6: You are not supposed to turn on your transponder when operating underneath the Schiphol TMA unless requested to do so by ATC.
Nothing to do with Mode-S. In fact, Mode-S will probably lead to less spurious TCAS alerts allowing us to keep our transponder on at all times. My last two flights in this area I was specifically asked by ATC to switch it on anyway.
Did wonder what effect a glider with Mode-S being winch launched would have.
You're not the first. Gliders are asked to leave transponders off until launched. I think it's in the AIC.
The Netherlands is actually not that bad in supporting GA. Schiphol airport is only marginally smaller than Heathrow and has more runways in more directions. Yet it is class C and transits are normally only refused if they actually interfere with departing and arriving traffic. Pick your route carefully and you can fly directly over the tower of Schiphol "not above 1000 feet".