[EDIT 2013 07 02]
Actually, it just partially works.
More later.
[/EDIT]
[EDIT 2013 07 03]
It seemed to work because half of the filter (in the example below) still worked : "IDItem in (167233)" worked, but not "check is null"
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Not sure why I never tried it, but it seems to work.
Just use one of these after your query (http://publib.boulder.ibm.com/infocenter/idshelp/v10/index.jsp?topic=/com.ibm.sqls.doc/sqls36.htm) :
I tried "--" and it works well.
E.g. : IDItem in (167233) | check is null -- THIS IS A TEST FILTER
Commenting filters can of course be useful when you're not sure what ID items are there, or with long/complex queries
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