Janna Q has 14 seconds cd on lvl 1 while Leona E has 13 seconds cd, so if you're lucky and Janna or her adc is out of position during the one second window you might have (assuming the Janna does not have a cdr advantage), then sure. It's kind of unlikely to happen assuming equally skilled players.
Considering 5% cdr advantage on Janna (if you have the common 0/17/13-esque Leona, 0/9/21 Janna) , Leona has a 0.3 second window to engage her while Janna's tornado is down.
You can technically abuse CDs in a 0.3 second window, but that means either Janna or her adc must be out of position while you and your adc are in position during that 0.3 second window. That's tough to fulfill.
A flash play can work, sure.
I'd say the match up pre-6 is mostly dependent on Janna messing up. Post-6 is another beast as Janna can't cancel the Leona ult.