I happened to run across a good example of productive use of combining marks, the Duden site (a great online dictionary for German). They use U+0323 ( ̣) COMBINING DOT BELOW to indicate the stress. Here is an example:
ụnterbuttern http://www.duden.de/rechtschreibung/unterbuttern They aren't, however, consistent; you also see underlining for stress. e̲i̲nschränken But not, interestingly, with the HTML underline, but with U+0332 ( ̲ ) COMBINING LOW LINE. Mark <https://google.com/+MarkDavis>
