Bert, I'm curious. Where else have you seen it used that way? I spent far too long searching the net trying to figure out more about this (because at this point I was intrigued) and mostly got frustrated with my own incompetence. A few sites on Indo-European did use this notation, but it's evidently not part of the current IPA system, which uses the combining ring for something else, and a little line (nÌ©) for syllabic consonants. Maybe it's a historical linguistics thing? (And is a syllabic consonant the same thing as a vocalic one?)
IPHIGENIE: Kann uns zum Vaterland die Fremde werden?
ARKAS: Und dir ist fremd das Vaterland geworden.
IPHIGENIE: Das ist's, warum mein blutend Herz nicht heilt.
(Goethe, Iphigenie auf Tauris)