If you enable the "Regular expression" option on AFilter, you can try with an expression like this: ^(?!.*DP.*) That would filter anything where DP occurs. This one should filter out exact matches (i.e. where attribute is exactly DP), though I'm not really a RegEx expert: ^(?!DP$).*$
You can try with the php function palo_getdata_export() (see the PHP API documentation in Knowledgebase for details). It allows you to set the option "ignore_empty", meaning it would return you a (potentially long...) list of cell coordinates with values which are not empty.