Besides, the ceremony won't last much longer than 20-30 minutes I'm sure. Tell the couple to skip any receiving lines and that you need to have them for 'couple photos immediately after', and I bet ya you'll have some pretty cool light for an other 20 minutes. Do them first, then a few of the bridal party and do the families inside .... Unless you can persuade them to do the first look version ....

You can make it work with your set up! Find some places to bounce the flash off the roof!