Simon,
Could you post the OQL constraints just to confirm that you don't have a syntax error that is causing that problem.
If your syntax is correct, it would unfortunately mean that you have found a bug in the reporting functionality of the platform.
DataSets can only be used and accessed through the build in reporting functionality. Those are not usable by any widget, currently not even from Java.
I wish I could give you a better answer but I am afraid that at this point your only option would be to use the SimpleChart or FusionChart widget with persistent entities.