There is a QA Add-in infrastructure in memoQ, just like your TM or MT Add-ins for memoQ, and we have tested in the past to try to create a plugin comparable to the one we have for Studio.
However, as of now, the memoQ QA plugin infrastructure does not provide enough context information to write a QA plugin comparable to the one we have for Studio.
So this prevents that we can develop an Xbench QA plugin that is useful enough. The reason is that the QA plugin infrastructure that Kilgray put in place was with a particular use case of batch QA, not for the use case of interactive QA in Xbench.
A while ago, Kilgray gathered information about the couple additions in context information that Xbench needs for a QA plugin. We understand that they might consider implementing them if their users ask them for an Xbench plugin comparable to the Studio one. On our side, any improvements in integration with other tools are top priority items, so we would react fast as soon as the required infrastructure is in place.
In the meantime, the integration with memoQ is as depicted in this video.