In some cases, normal properties in a manifest may require a message in the permissions prompt and it is redundant to have separate entries for a permission. A good example is the consideration of having a permission message for protocol_handlers (bug 1310427 has some discussion). We should have a standard mechanism for this, perhaps an additional key in the property [e.g. permission=true] so it can be added during parsing.
At this point we do this for the devtools page but its done through code. If this is something that keeps coming then we could re-open this. But hopefully we won't need it that much.