Line 6 Catalyst signal path question...

Is there anyway to 'bypass' the Catalyst amp 'block' and insert, via the effects loop, a modeler. I'm not talking about the power amp in method. I'd like to make use of the pre and post effects and have the option to use a Tonex pedal as an 'amp block' replacement.
The usual method is to just plug into the effects return while it’s still in effects loop mode. Sometimes you have to plug a “dummy” cable into the effects send and/or the main input.