Not really an answer but it was too long as a comment so. :-P
Switching IME should not be much of an effort. For me it's just a hotkey. It's supposed to be used this way (switched on whenever necessary). It might seem annoying when you list it like that but if you're used to it you barely notice (the switch is instant, and who complains about having to press the shift key to get upper case characters?).
More difficult problem is to know what to type :) to get a decent result from the IME you usually have to type whole words. If you just give it 'shi' you get a ton of possibilities...
If there aren't many files you could go with
zsh and use the TAB key to cycle through files. So these are tab completions for each single TAB press in
zsh (i. e.
cat TAB TAB TAB TAB TAB:)
cat ち.txt # dunno why it chose this
I'm not sure if you could use something like custom bash custom autocompletion filters to achieve the Pseudo-IME solution you wanted. There is
kakasi to convert kanji to kana, and converting ascii to kana is easy enough (converting kanji to ascii is ambiguous, as you can write ascii several ways to achieve the same result like
n could both be interpreted as
ん depending on context).
If there is an ASCII portion in the filename, you might also append that to narrow the choices down, in your example above you had a few
.sh files so (back in
zsh) you TAB for
*.sh and it won't offer you the others.
Or just use the good olde mouse select copy paste.