Hallo Marco,
AFAIK if you want to use a variable in all query level you would have to define it first, sth. like:
Declare variabe timestamp;
variable :='[%0]';
So you then can use this variable at the whole query level via @variable - if you dont declare it, it can be that HANA wont access it in subqueries - the exact reason however is beyond my current knowledge.
See also SAP Note 2082431 for more details regarding B1 Hana and FMS ( http://service.sap.com/sap/support/notes/2082431 ) and also SQL best practies on HANA ( Best Practices of SQL in SAP HANA )
Hope that helps!