It is possible. If you have a keyboard shortcut already set up to switch to Japanese, use that while in-game, then press Alt+` (same button as ~, not the key with the apostrophe/quotation mark) to switch to Japanese input (or back to English input). This is assuming you're using Windows with the Microsoft IME keyboard enabled. This works without a problem for me.
Just remember to switch back before trying to use any commands, otherwise you'll get stuff like "・えくいp" when you were trying to type /equip. (As I'm sure you're well aware.) ^^
Edited, Oct 17th 2010 6:45pm by Klarus