The easiest way would be to dump them manually to wav and burn those. The other way would be to get their file-length (time) and process that like ogg in your preferences, but I have no experience with flac, so I cannot really help you there. With ogg
oggtst outputs info, which is grepped for "length". That info is multiplied by something-or-other which then tells Nero how long the song is. Also you will need to have some dumping tool to dump the flac on-the-fly to stdout.
So, as I was saying .. the easiest way is to manually dump the file to wav