The button on the Japanese keyboard labelled 無変換 seems to do nothing with ibus-anthy. It is supposed to force non-conversion of a particular word.
Muhenkan only needs to work in preedit btw - I don't think reverse conversion is expected.
Here is an input example:
Ctrl-Space => anthy
a s a => あさ (pre-edit)
Henkan => 朝 (pre-edit)
Muhenkan => あさ (pre-edit)
Additionally, further presses of Muhenkan rotate through: hiragana, katakana, narrow katakana, double-width alpha, single-width ascii.
Fixed in ibus-anthy-1_1_0_20090402-1_fc11