Sorry if I add nothing new, I did not find the time to read the whole 
thread - but I would like to give my opinion on this subject, too. Also I 
have to beg your pardon for my bad English.

My problem with modern harpsichord compositions (and clavichord compositons 
as well) is, that the instrument is used there only as a special register 
or sound effect of keyboard (piano) in quite a lot of pieces (in all pieces 
I have seen up to now...).
The things which kept me busy over years, the techniques I had to learn, 
develop and cultivate to make my instrument (the harpsichord since 
childhood) sing, to give warmth and depth to the sound, to bring out the 
imagination of dynamics and different colours by the 'toucher' - all these 
things are ignored by this "modern" (?) music. (We should call 
it 'contemporary' instead of 'modern', because modern means "usual and 
accepted", doesnt'it?)

Of course a lot of important things connected with playing the harpsichord 
in an historical manner cannot be used in contemporary music without 
looking like a quotation or caricature, such as e.g. specific 
ornamentations.
[By the way: Concerning the discussion about "style" - in my opinion 
ornamentation is one of the most important indicators of style (not only in 
western music: look at classical North Indian music or Persian music!)]

Some of the 'modern' harpsichord compositions I have seen, are (I have to 
admit) very demanding in technique - but the technique which is demanded is 
mainly pianists technique and not a harpsichordists one.

So, up to now I have decided not to play 'modern' music - the music of 17th 
and 18th century (and sometimes 16th century) keeps me busy enough and 
gives more than enough space to express what I have to express.