Filter sp_BlitzIndex output
We recently added more options to the sp_BlitzIndex output, but the kind of housecleaning mode you're asking about isn't one of them, unfortunately. 

The closest it gets to that is to run it with @Mode = 4 (as of v3.0) or @Mode = 0 (prior to v3.0), which I suspect you're already doing. The benefit to using the v3.0 is you can use the @GetAllDatabases = 1 parameter to collect all the unused/dupes for the whole SQL Server instance in one pass. 

I suggest storing the output to a table and querying it for only the unused/dupes. Probably not the answer you were hoping for, but sp_BlitzIndex doesn't have that functionality built in.