I think you can do it. What you need to do is to plug one of your coax cables (either the OTA or the Cable one) into the co-ax input of the Slingbox and then plug the other lead into your VCR and use the composite output of the VCR to drive the composite input of the Slingbox. Then configure the 2 inputs accordingly.
Thank you so much for the reply, Alan. Never thought about using the VCR as a pass-through. I guess I'll use the VCR remote to control either the cable or the OTA.
My VCR is an Emerson brand. Assuming I will be controlling either the cable or OTA through the VCR, do you know whether or not the PRO-HD has a option for Emerson VCRs so that the IR Blaster will work on it?