Currently outside tools such as
- http://toolserver.org/~luxo/contributions/contributions.php (defunct)
- http://toolserver.org/~krinkle/MoreContributions/ (defunct)
are /were used to retrieve the equivalent of [[Special:Contributions]] for a particular IP or username across Wikimedia wikis. It should be possible to implement such functionality in an extension (similar to GlobalUsage for files). A user would be able to visit Special:Global Contributions (or some equivalent), input a username or IP, and retrieve the equivalent of [[Special:Contributions]], but in an aggregate form (on every wiki where the user or IP has been active).
Filing this is as a new extension request.