OP here. There most likely is. At least with uim, there seems to be settings I can see (in uim-pref-gtk) to enter kana directly. ("Default kana input method" : "Romaji", "Kana", "AZIK") And there's this thing on the toolbar (uim-toolbar-gtk) that seems switch between them too.

Or that's what I guess it's for anyway. Cause it doesn't really work here -- I can either set mlterm to use "XIM" as an input method and have a functioning tooobar with all the mojibake I could ever want; or a working "uim" input method with a cold, dead toolbar.

Oh well. At least uim works with hotkeys.