I have recently had success burning a Blu-ray disc of FCP produced video from Toast 9 after a ton of trial and error. I did not use Compressor. I did try it though. I created a new MPEG 2 setting in Compressor and chose Blu-ray as the stream usage to create a transport stream. Toast did burn the Blu-ray after reencoding, despite my telling it to never reencode. But here is what did work.
I was told by Toast Support that MPEG-4 AVC or H.264 AVC is best to save as from FCP. Don't know about the AVC because there is not such setting upon exporting using QuickTime Conversion in FCP. But I did plenty of tests with MPEG4 and H.264 QuickTimes from FCP. I have discovered that the root of all my problems is somehow tied to my using DVCPRO HD 720p in FCP. That native resolution is 960 x 720, not 1280 x 720. If I exported my timeline using current settings the resulting Quicktime file opens in Quicktime and reports a size of 1280 x 720. However, when taken into Toast, it reports it is 960 x 720. When trying to create a Blu-ray in toast from that file it either crashes at 97% with error -39 or it burns a disc that looks terrible (choppy playback or artifacts).
My success came when I luckily (accidentally) choose the export size of HD 1440 x 1080 16:9 within the Export-Movie Settings from FCP. Toast still reencoded it, even though I check never reencode. But the Blu-ray disc it created looks as good as my FCP timeline when played on my 56" DLP. So for now, this works for me and I'll stick to it.