You can, I've done it before. Make sure your manifest is set up right as outlined in the cookbook.
Thank you for encouraging me not to give up! I too got it working. My mistake was that Geometry/Size/Width and Geometry/Size/Height elements are required to exist -- though they may be set to zero. Example:
<Extension Id="com.test.invisible"> <DispatchInfo> <Resources> <MainPath>./invisibletest.html</MainPath> </Resources> <Lifecycle> <AutoVisible>false</AutoVisible> </Lifecycle> <UI> <Type>Custom</Type> <Geometry> <Size> <Height>0</Height> <Width>0</Width> </Size> </Geometry> </UI> </DispatchInfo> </Extension>