This thread is quite old, but I think it's never too late to give another answer.
Actually, this is possible with BGP passive mode. You don't specify "neighbor update-source".
The session must be initiated by the neighbor.
This is useful for peering with Google Global Cache servers, where "The BGP session established with GGC node is not used for traditional routing purposes.".
As you can see below, it was established between 1.1.1.65 (local VRRP address) and 1.1.1.94 (peer address).
The configuration:
interface TenGigabitEthernet1/2/0
ip address 1.1.1.67 255.255.255.224
vrrp 1 ip 1.1.1.65
vrrp 1 priority 150
!
router bgp 1
neighbor 1.1.1.94 remote-as 2
neighbor 1.1.1.94 transport connection-mode passive
address-family ipv4
neighbor 1.1.1.94 activate
!
Router#sh vrrp brief
Interface Grp Pri Time Own Pre State Master addr Group addr
Te1/2/0 1 150 3414 Y Master 1.1.1.67 1.1.1.65
Router#
Router#
Router#sh bgp nei 1.1.1.94 | i state|host:
BGP state = Established, up for 00:18:56
Do log neighbor state changes (via global configuration)
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Local host: 1.1.1.65, Local port: 179
Foreign host: 1.1.1.94, Foreign port: 39401
