RE the moveable grass field like in Arizona -- sure it would work here, but for a different purpose! Not necessarily to grow, since BMO Field will still be an open-air venue with natural light for the grass. Grass, especially with subfloor heating like BMO, grows fine outdoors in Toronto, for the times it's needed for footy. (Not sure why you said that grass needs to grow indoors in this area? It's actually quite hard to grow grass indoors, and will be a big issue for the Rogers Centre. One reason they will really need to baby it and protect it there; plus baseball puts much less wear on the grass than footy or especially CFL. But you can bet if they install grass in the Rogers Centre, they will leave the roof open as much as possible to get a bit of natural light & ventilation.)
That being said: many grass fields with significant roof structures do also have problems growing grass reliably, since a roof can put too much shade on the grass for it to grow well everywhere. Some venues have special artificial lighting systems ("grow lights") for the pitch as a result, to help the grass grow despite roof shading and poor weather. I'm sure Rogers Centre with grass will need lots of that; and it could help for BMO as well, especially with a new roof over the stands & as the MLS season has gotten longer.
However, the main reason to move the BMO grass pitch outside here would be to protect it during gridiron games & other events (and only during such events). Padding & plastic turf would be installed on the concrete subfloor for CFL when the grass is out. See the Veltins Arena (
http://en.wikipedia.org/wiki/Veltins-Arena), which actually has a full roof:
The biggest problems for that in Toronto are massive extra costs for the moveable pitch; plus that non-footy events (CFL; Winter Classic; CNE) want the space outside for parking, or directly for the events, so it would be hard to put the grass pitch there.
How about this idea, which we were just discussing with some structural engineers & engineering lab techies. They thought this would be possible. CFL will need retractable stands anyway due to the larger field size. That means for CFL events, the lowest available seating rows could start significantly higher than for footy. Instead of putting plastic turf directly on top of grass as Waggy suggested (bad for the grass; too soft for the plastic turf & will slide around) why not have a removable structure that spans the grass field, raising the floor, and provides a sturdy surface for plastic turf to be rolled out & anchored? The structure could be additionally supported by pins and subsurface sleeves within the grass surface, that won't be noticeable & can be filled with small grass plugs during footy games. The structure could even have grow lights on the bottom in case it needs to stay out for a few days for special occasions.
Wait a sec, I'm gonna patent that... ;) Won't be cheap either; but cheaper & easier than a moveable grass pitch. Basically, it's much easier to have removeable plastic turf, than removeable real grass; and CFL won't have as many games as there are footy games overall.