I work with a large entertainment company (french speaking clowns...rhymes with puree) based in Canada and all of their people traveling and working in Canada+US have two phones and two numbers...one US and one Canadian. Standard operating procedure. With the corporate sponsorship that they have (Delta, American Express, etc) they don't have a unified mobile phone solution, so I doubt that one exists for an individual.

When I'm working up there I'm issued a Canadian mobile for business and for personal stuff use Google Voice to retrieve voicemails and Skype to call people back stateside.

As an aside...two friends/coworkers have used the Verizon North America plan that you currently have and it is no longer available and they were actively sold new US-only contracts with XYZ perks for changing contracts so hold onto it for as long as you can.

None of this answers your question directly but if you find a solution please let me know because I know a number of people I work with would benefit.