Micros have a max empty weight beyond which you cannot fly the aircraft. A Eurostar (very popular micro selling in volume) is so close to this limit that it cannot be painted. There is no way you could fit a transponder, wiring and an aerial without making the aircraft illegal.
This argument, if true, affects absolutely anything one might want to fit into the said aircraft. One could not even fit a better quality seat belt buckle.
A GTX330 with the antenna and wiring is probably about 3kg.
It would be an utterly bizzare regulation. Why doesn't somebody press the (obviously stupid) manufacturer to increase the empty weight? Or replace a few bits with lighter ones - should be easy to fit titanium/magnesium parts on a homebuilt. Then, the issue will become a MTOW limit and be entirely in the hands of the pilots and how many £100 burgers they consume
More basically, why do people buy these machines when they are so obviously a regulatory trap / dead-end? Are the buyers unaware of these issues? Even if Mode S remains for ever non mandatory for certain bits of airspace, not being able to fit it makes the machine hugely restricted.
Does the UK dealer for these spell out that in the near future, large chunks of European airspace will be closed to them? If he doesn't, he is likely to get sued. I know I could not sell a plane which on the basis of empty weight will be illegal to fly in a lot of perfectly OK VFR airspace, unless I spelt this out in the sale contract.
Whatever one thinks of Mode S, the writing has been on the wall for years and nobody can pretend they didn't know. The European Mode S war has been lost; all that remains is some haggling over which bits of low level airspace will remain open in which countries.
Presumably if somebody wants to fit a GPS, it has to be a handheld, on a clip-on bracket, so they can pretend it is portable and does not add to the empty weight.