I'm in the process of renovation of a 1945 house. Originally built with a flat roof, tar and gravel. At some point a gable roof was constructed over the original flat roof. Space between the ceilings and flat roof is 10 inches, there is insulation in that space ,about 2 inches of paper backed fiberglass. I would like to add more insulation with out removing the existing ceilings. My plan was to blow in cellulose on top of the tar/gravel roof. Does this make sense.
I'd blow cellulose into the 10' space, from above. I'd worry about moisture and rodent problems with cellulose between the old tar & gravel and the rafters. The tar & gravel makes for a moisture barrier you don't really want.
Check your venting in this attic space, and consider a humidity controlled fan. You probably have K&T wiring in the 10' space, if so see https://diy.stackexchange.com/a/20279/5960
This does make complete sense, you'd have a nice layer of insulation there if you fill that void. As it stands, that pocket might be in danger of forming condensation when your inside temp differs greatly from your attic temp.