Hello Roland,
what you need, is routing a call based on device states. This feature is currently not included into FreePBX (but I hope, it will be in the future).
Maybe, this thread may help you: Call Routing on states. You’ll have to change my code a bit.
I would do a “gotoif” in custom context, if all of your ring group members are busy, to a second ring group or extension, which can be picked up via BLF.
A direct pickup of a ring group is possible - you have already mentioned the way to do it. To get a BLF for a ring group, you need to create a hint in custom context for your ring group.
I.e.: exten => 604,hint,SIP/900&SIP/901&SIP/918
where 604 is your ring group extension, and SIP/900 etc are your ring group members, which you want to monitor. Works fine for me!