Pick the transport based on the destination host, not domain?

[Darren Pilgrim]

I've run into a problem with a hosting service's IPv6 connectivity. 
Their IPv6 broken such that they get odd transient failures.  Normally 
not a problem, but their anti-spam appliance or whatever they're using 
in front of their mail servers hard-bounces on those failures instead of 
following the established-since-forever practice of soft-bouncing on 
such issues.

Originally I had only one domain using this hoster, so for that one 
domain I simply added a transport map that points the domain at the an 
extra smtp transport that has -o inet_protocols=ipv4.

But now I have a second such doamin, and I'd like to head-off a 
maintenance problem.  All such domains use the same set of MXes, so it's 
an obvious pattern to switch transports if the next hop is one of the 
offending MXes.

Can Postfix even do this?  A read through the relevant docs indicates 
those transport lookups are based solely on recipient domain.

If it can't, is there something out there that can do this using the 
socketmap interface?  If not, I'll write one.