Right now, itemid has to be unique (in a search area) and also has to be an int.

The severely limits the ability to index certain types of records, like email messages, because you can't index the message multiple times in references to different owneruserids.

Ideally I would be able to have an itemid like 'userid:messageid'.

After chatting in the dev chat, there is no real immediate concern with changing it to a string, it just needs to be looked into deeper.