Since version 1.98 XnView is able to show the different pages of .mpo files. If you add an embedded comment to a .mpo file the buttons to select the pages are disabled. It is not possible to select the pages any longer.
I tried with 1.99.1. It is a bit weird. At first glance it is possible to set comments (descript.ion) and embedded comments (ctrl D). It appears the mpo's are left intact. XNView itself still recognizes two frames but doesn't show the second frame any longer .