on Mac systems one can use so-called flying (or, in Mac speech, combining) accents, but the quality of the output depends on the font used. Gentium Plus and Brill are quite good, but I seem to remember that there are problems with some more outlandish combinations. At least, e.g., r + combining ring below + combining macron + combining acute accent works well with both fonts, and even with Times, though less perfectly.

Best wishes,

Sven