How about let routers accumulate (with XOR or something like that) link CRC (or some psuedo random number involving link specific numbers such as MAC addresses of a hop) into (so far useless) flow label field to let upper layers notice the path change to trigger the transport layer?
Similar to the record route option.
The essential difference is to resuse a fixed length field without burdening routers.
I myself am afraid that it is too late to change IPv6 router behaviour. So, I never proposed any multihoming solution involving router changes.
Another way to detect path changes, but not a very reliable one, is to monitor the hop limit value in incoming packets. And of course the round trip time.
However, I know transport people are actively proposing to change router behaviour to help TCP and they have better chance with IPv6. :-)
Hm, maybe we can make a deal so that they change TCP behavior to help routers...
I'm, and most of transport layer people, are talking about behaviour at the IP layer, even though the bhaviour may be meaningful for connection oriented transport layers.