Moving to authenticated API calls to each wiki (ouch!) to retrieve global contributions would ensure this tool remains functional.

Yeah, that would be the only way... unless we get clearance to connect to the analytics replicas. If that can be done, then we can still provide roughly the same speeds we have now, and very little additional engineering would be needed (as it's mostly just a change in what databases we connect to). I'll get in touch with the Data Engineering team to see about feasibility. I suspect the main barrier will be a security review, and with XTools the size it is, that could take quite a while. It would be pretty awesome to have this access, though, as we could basically build a global CheckUser tool!

I'm also going to tag Tool-Global-user-contributions as that tool is also effected by IP masking in the same way. (It could be a separate task, but I'm assuming in the end both tools will have the same solution).

Note that all of this is merely a bandage. GUC and Global Contribs were created because we lack a solution in MediaWiki. With IP masking on the horizon, I think now is as good of a time as ever to get started on a proper global contribs tool in MediaWiki.