If you have a straightforward configuration of order types, you can do &t=testorder and &t=drugorder. This will trigger a REST SubclassHandler which corresponds to filtering on OrderType.javaClassName.
If you have a more complex configuration (e.g. Radiology Order and Lab Test Order both use the TestOrder java class under the hood) then I don’t think the REST API lets you filter. (I haven’t looked in the code for this, because I assume it’s not your situation, but if it is, I’ll look.)
I just tested this out (we’re still on RestWS 2.14) and it doesn’t seem to be working properly. Using t=testorder and t=drugorder returns a set of all orders, in this case, we’re testing on a patient with a single testorder. Note that when we change the string, to say t=test, no results are returned.
We are planning to update the module to latest stable release today. We’ll provide an update.