Hi,

I think the best guide closest to your description is here : http://kb.asipto.com/asterisk:realtime:kamailio-4.0.x-asterisk-11.3.0-astdb

Here is what you need to do. (Besides mentioning what you tried and what problems were faced).

1 - Configure kamailio to use the DB schema where your users are stored with their password and PBX to use info.
2 - Point Phones to REGISTER to your kamailio.
3 - When a User makes a call execute query in Kamailio to find what client this user belongs to  and what Asterisk it should be routed to.
4 - Send Calls to the selected Asterisk. 

You can further add Memcache to save your DB query in step-3. 

Good Part:
1 - Kamailio Authenticates all users and calls.
2 - One Public IP to point all domains/PBX tenants to. 
3 - Use RTPproxy to bridge media to Asterisks and you can shift your Asterisks on Private Subnet too.(depends on your design)
4 - Sending a hand crafted REGISTER to Asterisk makes asterisk aware of the device state and hence BLF/MWI are handled by Asterisk.

Less Good Part: (As I see it)

Kamailio sends REGISTER packet to just one Asterisk ! thereby only one server out of pool is aware of the device states. It can be resolved by extra effort required as following:

      a) Yes we can use Dispatcher and send to failover/loadbalanced asterisks in the pool
      b) A script of some sorts can be written and started in asterisk servers to share device states/hints and \
          hence all asterisk servers in pool know whats going on. (I haven't tried it myself)
      c) REGFWD route can be blocked and BLF, MWI are handled solely by Kamailio.  ( I personally had rough time with this mostly due to different standards from IP Phones)

I'd love to hear other valuable suggestions and experiences.

Regards,
Sammy