I added as-path-exclude as an option to set in a route-map, like as-path-prepend. This is supported by Quagga but not mentioned in the documentation. Pull request https://github.com/vyos/vyatta-cfg-quagga/pull/11

My usecase is as follows:

7224 A           7224 C ----- D 65506 VyOS
      \           /
       B 64512 VyOS

I wanted C to install a route to A through B so that it (C) would then announce it to D.
A and C don't exchange routes between themselves. I control B and D.

C wouldn't install the route to A as it contained its own AS in the path. So using as-path-exclude I stripped off A's AS, and now it works.