Additional user permission levels

Hi,

The user permission levels currently include
- View Invited Services Only
- View & Edit all Services and Songs
This is a bit "all or nothing" - could something in between be added?
With different (possibly visiting) preachers each week,
- View & Edit invited Services Only
would allow them to create and update the particular services they are leading, but without the risk of them inadvertently changing something on one that they're not involved in (e.g. because they click the wrong week in the Services list).
And/or
- View all Services and Songs & Edit invited Services
would again prevent changes to a service someone's not involved in, but still allow them to view others in order to see what our usual running order looks like.

Regards

Andy