We are going to develop another Action Plugin that will count incrementally archived data. We will truncate piwik_log_link_visit_action table after archiving and accepted incorrect unique visitor number (we will develop another method to count unique vistor).
But there is some case that
$rowFound->sumSubtable(Piwik_DataTable_Manager::getInstance()->getTable($idSubTable)); cause error because we can't find the Table of idSubTable because it's loaded yet.
To overcome this problem I have add a try catch statement surrounding.
But the better way could be change the sumSubtable function and it's parameter.
Can you please submit a patch of what would work for you? http://piwik.org/participate/development-process/#toc-how-to-submit-a-patch