yep, press & hold is possible. this should be mode switchable too, tied to the press & release action. so when in next/prev it would be ff/rew and when in scrolling up/down it would be fodler_in/folder_back.
in fact, this doesn't need to be ibus specific. so I'll start by having an ibus module to talk to it, which can later be replaced with a generic serial module (fo those with custom interfaces)