It looks like sp_executesql doesn't like such complex parameter. Let's try to get it work w/o using parameters.

...
set @sql = 'SELECT * FROM OpenRowset(''SQLOLEDB'', ''z'';''z'';''zz'', +
REPLACE(@Query,'''', '''''') + ''')'

--select @sql

exec sp_executesql @sql

I got it work on Pubs..Sales table. Here's my test code for reference.

declare @Query nvarchar(400)
declare @SQL nvarchar(400)
set @Query = 'SELECT * FROM Pubs..Stores where state = ''CA'''
set @sql = 'SELECT * FROM OpenRowset(''SQLOLEDB.1'', ''zzz''; ''zz''; ''z'', ''' + 
     REPLACE(@Query,'''', '''''') + ''')'
exec sp_executesql @sql

>Here is what I got now:
>
>

declare @datehb datetime
>declare @Query varchar(200)
>declare @SQL nvarchar(200)
>declare @ParmDefinition nvarchar(200)
>
>set @datehb = (select max(date) from histobarraCNE3)
>set @ParmDefinition = '@QueryParm varchar(200)'
>set @Query = 'SELECT * FROM RQuant.dbo.histobarrause3 where date> ''' +  
     cast(datepart(yy,@datehb) as varchar(4)) + '-' + 
      cast(datepart(mm,@datehb) as varchar(2)) + '-' + 
     cast(datepart(dd,@datehb) AS varchar(2)) + ''''
>--select @query
>
>set @sql = 'SELECT * FROM OpenRowset(''SQLOLEDB'', ''z'';''z'';''zz'', @QueryParam) '
>--select @sql
>
>exec sp_executesql @sql, @ParmDefinition , @QueryParam = @Query

>
>The error message I have now is:
>Server: Msg 170, Level 15, State 1, Line 1
>Line 1: Incorrect syntax near '@QueryParam'.
>
>Other ideas? You must be closed to a solution now.
>
< snip >