I think some of the 'absolutely no amplification' rules are really about setting a precedent and avoiding escalation. Belt and braces! We know in principle that almost everyone will just use very discrete amplification for balance or whatever, but it only takes a few people to start routinely plugging in, a few musicians more used to open-mics to start turning up, and before you know it the whole atmosphere has changed.

That said, when I was playing very quiet fiddle (not folk, for free improvisation/avant garde stuff) I used a £15 clip-on mic from Maplins (I think the brand was called 'Yoga') through a tiny 1 watt Danelectro Honeytone amplifier clipped to my belt (they're about £30 new). Unless anyone actually saw the amp and clip-on mic, nobody even knew I was amplified. But they heard what I was playing.

If you must amplify, go for the minimum amount of watts possible; really, other musicians ought to be playing quieter to allow you to be heard; and the audience should be listening harder! that's presumably how it worked before amplification existed.