What character is transliterated from cuneiform Hittite as -ah- (with
a breve below the 'h')?

From the spellings given in p68 of 'The World's Writing Systems' for
the 'fire' word (pa-ah-hur etc.), it is U+12134 CUNEIFORM SIGN HI
TIMES NUN (Old Borger 398), but from the charts in Held 1988 it looks
more like U+1202A CUNEIFORM SIGN ALEPH (Old Borger 397), the match
being with the neo-Babylonian form given in Labat's charts. There is
a Hittite font that encodes the Hittite character as U+1202A.