As for most things Greek, its not what you know but who you know – at least that's how things used to be before they started sacking everyone....
You should be ok, Graham. A BR20, even with an non-standard bowsprit, can't be much more than 6.5m LOA, "Four Sisters" also beats the tax threshold and my Deben Lugger hybrid (my present "Greek" boat) is well under the 7m mark, even with bowsprit and bumkin!
I'm hoping that my friends who run a Greek registered charter boat
http://www.paxoslife.com/goldenodysseus.htm won't be affected too badly ( I haven't read the relevant legislation yet ).
Perhaps this tax, whatever its motivation, won't be all bad, especially if some of the proceeds are earmarked for environmental improvements rather than going straight back into the Bundesbank or wherever it is they have to pay the interest on all the bail-out loans.
For example, I would think that avoiding major ports and marinas and anchoring “wild” will no longer be the chosen option for the hordes of Gin Palaces (mostly Italian) that appear in summer – which might encourage them to use the facilities and stop 'em pumping out their holding tanks where ever they feel like it.
The corporate superyacht owners wont give a damn, of course..that's what accountants are for... and, for once, the little guys like us get off unscathed!
(Unlike the UK where everywhere you go someone appears wanting harbour dues, parking fees, slipway charges, etc. )