Unfortunately, the Intuit Anywhere APIs really are this crappy at the moment. :-(
Here is the list of filter operations that are supported for strings: https://ipp.developer.intuit.com/0010_Intuit_Partner_Platform/0050_Data_Services/0400_QuickBooks_Online/0100_Calling_Data_Services/0030_Retrieving_Objects#Supported_Filter_Operators
Note that the only one supported is:
:EQUALS:
Ick!
Fortunately, Intuit is hard at work on the v3 APIs. It was rumored that v3 will support better filtering.
Unfortunately, Intuit is doing their usual thing and not involving developers in the v3 dev process anymore, so we really won't know whether or not v3 is going to suck until it's actually released. sigh You'll notice the last update about v3 data services was in October by Wei... unfortunately Wei isn't even on that team at Intuit anymore, so who knows what the status is, or even if there's anyone at all working on v3 anymore at Intuit.