I wouldn't think so, but as was mentioned above, the sticking point may not necessarily be money, but riders. GO is already pretty full, and cramming TTC riders onto GO trains won't exactly help that.
I would venture to say the reason GO is pursuing co-fares in suburban areas is to reduce the amount of parking they need to provide, while still keeping ridership high. I would venture to say that they amount they have to dish out per rider using co-fare is roughly the same amount as it takes for GO to provide a parking spot, perhaps even less. Giving what basically amounts to subsidies to 905 transit systems that are already not very well used is a heck of a lot cheaper than building a parking structure.
Since none of GO's Toronto stations (with the exception of maybe Guildwood and Rouge Hill) have any substantial parking, that subsidy in order to reduce parking is pretty much pointless.