a) I hear you would like to be able to distribute your patch. b) A Pd patch is not easily turned into closed-source for technical reasons.
exactly (that's why I almost went to max/msp for this). doesn't matter
which license it has, the patch itself won't be "locked".
I was thinking of instead the BSD to use a CC license, the "Attribution
No Derivatives" one - http://creativecommons.org/about/licenses/.Hm.. this makes the GPL unsuitable again. However, I do believe that the CC licenses are more suitable for artistic works (not that a computer program also can be an artistic work!), where a 'no derivatives' clause makes more sense, because a derivative work might change the context of the original oeuvre thus possibly changing the meaning of the original work completely, which the original creator might like to avoid.
that is true, the spirit of CC might not apply here 100%. I know CC better
than other licenses because they have a very clear presentation, it's easy
to read in 5m.
In terms of software, since you have only little control over how people adapt your patch for their own use (and of course, they probably _would_ like to change at least a tiny thing and forcing them to ask you first adds an imho unnecessary layer of bureaucracy), I suggest that the best options for you (and also for your users) is to 'pro-actively' allow them to make changes. The most important part is the 'attribution' clause, which is covered by both CC and GPL, iirc.
for those who want to change something - which would be the users of this
list, who, btw, almost for sure won't be needing this patch - I would have
nothing against changes (either small ones or putting in new features), as
long as attribution would be there, and they wouldn't disseminate the
patch as the original one [although for now I would prefer that no other
version but the original one is around].
In a real world perspective, I'm almost 100% sure that the people who can
work inside the patch (i.e. all of you) won't be interested in doing so,
because it's a field that doesn't interest you that much. Since anyway I
can't stop anyone from changing anything, I'm sure if someone would need
to adapt something for its personal use, they won't be waiting for a mail
reply from me.