Iʼm sold, I think... (U+02bc isn't really intended for such use, but until there's a proper alternative, other than U+2019 or U+0027, I'm using it)[1].
(in ~/.XCompose)
include "%L"
<Multi_key> <apostrophe> <minus> : "ʼ" U02BC # MODIFIER LETTER APOSTROPHE
[1] A potential problem with U+0027 is that low-ascii ' and " have uses for demarcating things (like attribute values in html, most popularly), so if you're editing anything that uses ' for markup, you can't search and replace based on ' anymore.
