Blowing in cellulose w/o ventilation is what is called a "Hot Roof". Hot roof is a commnon practice when using foam boards over a timber-framed roof. it can also be done with cellulose. I've used such an application many times w/o problems. There are a few important factors to make it work properly.
A significant property that cellulose has is that if it is packed to a significant density (Commonly called dense pack) it vertually elliminates air from being to more through it (no matter how much you pack fiberglass, it will never accomplish this). If we acheive this pack, we can have an unvented roof. This is because that the packed cellulose virtually eliminates air leakage from your living space into the attic area, the primary mechinism for transporting moisture into your attic.
As far as I understand it, your ceiling in this area is drywall. This being the case, many paints qualify as a vapor barrier (some primers are marketed as vapor barriers). Installing a vapor barrier is as easy as repainting.
Again, don't be over concerned about about the ventilation. To many builders this seems to be the "Holy Grail". If you eliminate the reason why you need ventilation, you eliminate the need for ventilation.