I notice from document N2626 posted on the WG2 website today
(http://std.dkuug.dk/JTC1/SC2/WG2/docs/n2626.pdf) that China is proposing a
whole host of "IPA Extensions and Combining Diacritical Marks", including 225
"five-degree" tone values, which cover both left-stemmed and right-stemmed tone
marks, as well as most of the possible combinations of complex stemmed tone
marks that you have suggested could be represented by means of ligatures (and
indeed which are so represented in your excellant DoulosSIL font).
An interesting document. No samples, of course. >:-(
There are some characters there already in the standard; I'll do some work on making a unification list before the WG2 meeting.
--
Michael Everson * * Everson Typography * * http://www.evertype.com
