Intuitiveness helps...

but consider how most people use their empeg most of the time:
1) Play one specific playlist - album, artist, genre, whatever.
2) Down, Down, Down...(whole player)

Maybe I'm wrong, but starting from one track/playlist, and then enqueuing as the mood suits 'Jukebox mode', is probably not used anywhere near as much, so whatever change is made needs to take account of this and still allow switching between the 2 above scenarios easily.
Making it use the current search mode wouldn't work very well - the only way to have the 'easy playlists' is to leave search mode in 'replace', but when searching, it is most useful in either 'insert' or 'append'.
If both search and playlists had their own mode setting, then the scheme would work well.

As for mode switching without the remote, your other suggestion would work usability-wise, and IIRC be the same as the search functionality.