Why are you guys overcomplicating this? Like
@nfitz said, it would be just like the agreement that GO already has with the TTC, which seems to work fine.
If there is a major unplanned service disruption on a GO line, GO Transit contacts the TTC and invokes the agreement. So for the duration of the disruption on the GO line, TTC staff accept GO Transit proof of payment. The inverse is true when the subway shuts down unexpectedly. For example, one time I was heading west on Line 2 and the line shut down at Jane the operator informed passengers that they were now able to ride GO Transit for free to bypass the closure, encouraging people to alight at Dundas West.
Example from Kennedy Station page on GO Transit website:
View attachment 469734
The agreement only applies while there is an major unplanned service disruption, and for the purpose of avoiding that disruption. There are very few people who will think "oh boy there's a major service disruption, let me go buy and refund a VIA ticket so I can join the chaos on the corresponding GO train!".