asterisk users - Jun 2014 - Multiple Servers: Multiple Peers: call-limit

I would like to enforce call-limit across multiple servers.  Is there
any way to pass a call-limit variable between servers 01 & 02, as shown
below?  Use a global call-limit between multiple servers and peer
connections.

A -- 01 -- Z
A -- 02 -- Z

A is using round-robin to reach Z, but in the event that 01 or 02 fail,
I want the full call-limit available to A.  The call-limit is only
applied between A and the middle servers.  For the sake of discussion,
let's say call-limit=10 for both, and the total limit should also be 10.

Since my round-robin configuration will fall-back to the other server,
calls can reach a maximum of 20.  Not a state I want to allow.

#server_a_extensions.conf
[SERVER01]
exten => _X.,1,NoOp(Use: First Server)
 same => n,Dial(SIP/A-to-01-to-Z/${EXTEN})
 same =>
n,GotoIf($["${DIALSTATUS}"="CHANUNAVAIL"]?SERVER02,${EXTEN},1)
 same => n,NoOp(yes, it's incomplete)

[SERVER02]
exten => _X.,1,NoOp(Use: Second Server)
 same => n,Dial(SIP/A-to-02-to-Z/${EXTEN})
 same =>
n,GotoIf($["${DIALSTATUS}"="CHANUNAVAIL"]?SERVER01,${EXTEN},1)
 same => n,NoOp(yes, it's incomplete)

I've though about passing the variable between the middle servers in a
SIP message, side communication channel.  But, hoping there might be a
simpler solution.


Sincerely,
Brian LaVallee