1) Add one new character, ZERO WIDTH INVISIBLE LETTER,>
I strongly prefer solution 1 because it is fully general with a minimum of effort added. It can also handle TeX's tie accent.
TeX's tie accent is an inverted right shifted breve above -- that's how it is implemented in TeX and METAFONT by Donald Knuth. It has the width of a
normal accent, but the glyph hangs out of its bounding box such that it is placed between two letters. The thing is used in some transliteration of russian, where the letter <ya> is transcribed as \t{\ia}, i. e. an inverted breve placed between a dotless i (\i) and a. A sample can be found in Donald E. Knuth, the TeXbook.
What's wrong about U+0131 U+0361 U+0061? I believe U+0361 is intended for ties.
On Slavic transliteration, see, for example, the nice GIFs for IE, IU and IA at http://infoshare1.princeton.edu/katmandu/sgman/trrus.html. I think LOC standard is to use the double-width glyph. I guess Knuth used a normal inverted breve either because he ran out of space in his fonts or because he didn't really care about the glyph (it being extremely rare, after all) and just inverted his existing breve. Anyhow, this is just a glyph difference.
I could imagine adding a ZERO WIDTH INVISIBLE LETTER means asking for trouble with non-Latin scripts.
Solution 2) is also a good one and it can be extended easily to the case of TeX's tie accent by adding a second character, COMBINING RIGHT SHIFTED INVERTED BREVE ABOVE, to the UCS.
See above on U+0361. :)
Solution 3) is ad hoc and will probably open the door for dozens of other candiates (like the tied ia).
On ia see above.
Solution 4) Use U+035D instead. Even more so if the character is not a superscript u, but a combining breve in between. Have mapmakers use an appropriate glyph; they tend to use special fonts anyway.
Philipp
