What issue? Root outside the jail is implicitly trusted by the jail so why would you hide resources (in this case jailed processes) from the "true" root? Jails can't see each others processes. The only case I can think of how one jail could affect an other is if they are connected by a unix domain socket in that case you could probably send a process descriptor from one jail to the other and the capability represented by the descriptor should cross the jail boundary. This would require the sending to sendmsg() the descriptor on purpose and I wouldn't consider it a security problem, because both jail have to cooperate to create this condition and be configured to share an existing connected unix domain socket a shared writable file system subtree (e.g. through nullfs what Linux calls a bind mount).

Yes you can limit resources per jail through hierarchical resource limits.

Does this answer your questions or did I missunderstand your question?