Hi, I have exactly the same music collection, so what I did, is just create two libraries, one MP3 and the Other FLAC, when I want to listen to the flacs, i just load that Libray, is it not that fast but works for me. I do this when I want to listen to hi-res music at very quite time on the day. All my flacs are been converted to Mp3 to complement all my mp3 collection, of course at 320kbps. Other way to deal is to create different views within the library just set the rules for that view to load the flac type and that is all you need to do.
John