N.B. The email exchange below may have been edited, e.g. to remove content not essential to the main point(s) or to standardize English spelling/grammar.
I was looking through Unicode for a way to distinguish retroflex 'l' from vocalic 'l'. I would like to write the vocalic liquids with subscript circle, and the retroflex consonants consistently with subscript dot. There is an r+circle but no l+circle. Am I missing something?
What you seem to be missing is the fact that Unicode does not attempt to provide Latin letters with all diacritics or combinations thereof. I don't know how they chose what they chose, but others must be composed with non-spacing diacritics.
I assume, by "subscript circle," you refer to what in Unicode and elsewhere is called "ring below." Unicode has the following:
U+0325 : combining ring below, a.k.a. non-spacing ring below
So enter this character after L to get your L with ring below. FYI there is a combining dot below, and numerous others. In order to view some of these characters, you may need a hefty Unicode font. I can't tell what your platform is, but in Windows the usual Arial and Times New Roman fonts lack "ring below." However, Lucida Sans Unicode has it. If you have a Mac with OS X 10.2 or above, it should [I assume] be in Lucida Grande. For the names of a few other [Unicode-compliant] fonts, see our note re: Text Encodings.