I assumed DC-Bass Source included everything it needed, but I see now that OptimFROG is proprietary, so maybe the author(s) of DC-Bass Source can't legally distribute OptimFROG.dll.
Is it okay to leave the "FLAC" labels in the cue sheet or is there another reason why I should leave them as "WAVE"?
You're confusing things. EAC is a backup program to convert your audio cds to any audio format you want; FLAC(image+cue), or FLAC(tracks) for instance. ImgBurn is for the other way around, to burn the FLAC(image+cue) to an audio cd again (amongst many other features). The latter is what you want, right?Install DC-Bass Source (Mod), or any other DirectShow FLAC decoder, create fully working cue-sheets (Only change .wav to .flac. Don't touch WAVE), and you should be able to open the cue-files in ImgBurn.