Doesn't work.

Voting was mandatory in many (most?) Cantons until the '60ies and '70ies or so, with a fine (5-10 CHF as I recall, that's like 20-50 today) for those who couldn't be bothered. Didn't work, ever more people abstained anyway, so the fines got dropped. IIRC Schaffhausen is the only exception nowadays.

It's also a fundamental matter:
Since the voter is the final arbiter it must be their sole decision whether they use that right or not - it's a right, not an obligation. Given the differences between UK and CH, I can see though that their stance may differ, as they're not the final arbiter in the UK. However, if participation were mandatory the outcome should also be binding.

And what's the difference between "can't be bothered to return the envelope" and returning it empty, what different conclusion can be drawn?