A major issue for users coming from StatusNet to pump.io is that pump.io does not support the Twitter-like API described here: http://status.net/wiki/Twitter-compatible_API If client developers have not or will not port their clients, the user is out of luck.
One way to mitigate this is to have an API facade that translates calls to the Twitter API to calls to the pump.io API, and vice versa. Users would log into the site (I have apifacade.com, so I'm going to use that name for now) and create a new account, e.g. email@example.com. They can then use "apifacade.com" as their "StatusNet" site name for clients that only support the Twitter API.
client <- Twitter API -> apifacade.com <- pump.io API -> user's pump.io site
Obviously functionality will be limited, and there will be a performance hit, but it will be better than nothing.
Note that this won't be a feature of pump.io, but a separate site. There will be naming conflicts (firstname.lastname@example.org and email@example.com won't both be able to get firstname.lastname@example.org), but people can work around it.
After a few months, it's become clear that people are fine working with the new API.
I'm not sure I'm ever going to do this kind of service. But I'll leave it open in case the idea comes up again.
Would it be possible to hijack /etc/hosts and use existing twitter clients to post elsewhere ?