"Can anyone confirm whether or not EX1 footage can be captured direct-to-computer through the USB2 output? "
NO, USB sends packets of data and it is only used to transfer data. Firewire or SDI send streams of data. I couldn't imagine not having enough capacity using the cards. You would need two of them and as one is getting full you can switch to the second while you or someone else transfers the data to your MBP.I know these SXS card are expensive but here is a link to something you will find as an alternative.
Read this thread.
http://forums.creativecow.net/thread/142/861674#861767
Good Luck
Michael Palmer