To be honest where I need to send mass SMS I use gateways such as Clickatell or BulkSMS. Yes they cost a couple of pennies per text, but its guaranteed delivery, full delivery reports and comes with an API that works extremely well.
I even wrapped it all up in windows app (using Ubot) to make it easy enough for anyone to use!
If you want free, I always feel that there is no guarantee that the text will get there - unfortunately I don't have a Google Voice account as I am not in the US, but I guess that would be easy enough to automate as well.
Might proxy up and grab an account to see what happens!