I guess the indieweb approach would be to post everything to your own site, and syndicate out to the other services (or the reverse). That still requires accounts on the other services though. With the rel=me you can at least say, I vouch that this is me on the other service.
Don't know if that really solves the exact problem here, but doesn't feel too dissimilar to what Matt is describing. One unified identity interacting with multiple clients.