Are there plans to implement a page scrubber for FXL in Readium?
Not at this time. By Readium, I assume you mean the Readium Chrome app? Bear in mind that Readium is an open-source effort, so we need volunteers to implement such features and at present we do not have a volunteer for this feature.
Actually, I mean the Readium SDK.
Ah, well. This is not really an SDK-level functionality (though I suppose there might be some minor aspects that would make it easier, such as grabbing snapshots of pages). But it is primarily an application level functionality.