Marco
Here's a further solution to your problem which demonstrates how to use \tweak to change the shape of one tie within a chord. Thanks to Han-Wen for showing how this should be done. I've made a note to add this to the docs.
\new Staff <<
\key f \major \time 6/8
\new Voice {
\voiceOne
\relative c' {
<fis~ d'~>4. <fis d'>4 <a e'>8
<fis
-\tweak #'control-points
#'((1.75 . -0.5) (2.5 . 0.75) (8 . 2) (13.5 . -1.2))
-~ d'~>4.
<fis d'>4 <a e'>8
}
}
\new Voice {
\voiceTwo
\relative c'' {
a8 a16 a a8 a a a
a8 a16 a a8 a a a
}
}
That's nifty.
Would it be worth a page somewhere in manual just on uses of \tweak? Notehead properties within a chord are the obvious example. Formatting nested tuplet brackets that begin at the same moment is another really good use of \tweak. And now the tie control points.
Maybe the different \tweaks should appear in the relevant sections of the manual concerning the object of the tweak -- in chords, tuplets, ties, etc. But it just seems like a small page on the uses of \tweak could be structurally enlightening.