This one has hit me years ago (I gave up on CMIS at that time). As of CMIS 1.1, there is this optional cmis:item
. Have a look at http://docs.oasis-open.org/cmis/CMIS/v1.1/CMIS-v1.1.html#x1-220002 . You may be lucky and find that working with Alfresco. It might be required to use the v.1.1 implementation of the spec.
If that does not work, I guess your only option is to abondon CMIS for this query and either use another search service or roll your own.
Good luck ! ;)