[LAU] Ardour-Rosegarden mixdown

Hi all, I'd asked on this list about this a few months back and didn't receive any replies, and then wrote again but the old server bounced the message. So, I apologize for any excessive repeats in advance. When I mix my tracks down from Rosegarden 5.1 to Ardour .99.3, I've been using a realtime record to a stereo Ardour track. However, I'd like to do a non-realtime mixdown. I've been told that Rosegarden and Ardour are supposed to sync during an export if Rosegarden is an input to a bus, and this will cause JACK to snap into a non-realtime mode. But, when I try this, it just exports silence, and Rosegarden doesn't seem to ever react. Has anyone tried this? Any success? Thanks in advance, Louis