Don't believe for a minute that inland is "actually faster processing" - it's almost never the better option for applying, especially if you're from the States. Inland first stage approval (and the ability to apply for an open work permit afterwards) takes at least six months, and that's only when there are no issues with your application, and then it's another 6-12 months before you're finalized. But if Buffalo has requested an interview, your inland application would have been subject to that, too, and it would not have happened in Vegreville. They would have transferred your application to the local CIC office closest to where you're staying and, if that office is busy (most of them are) you could have ended up waiting for up to two years (without first stage approval or an OWP) before you finally got your interview. In addition, if the application was refused, your sponsor would have no right to appeal the refusal and you'd be out of Canada - having to start all over again (if you could overcome the reason for refusal) with an outland application anyway.
Seriously, you did not make a mistake applying outland. Are you processing through Buffalo because you're from the States? If so, which State? And do you have any idea why Buffalo has requested an interview? When did your application get to Buffalo?
To answer your original question, though: no, you're not eligible to apply for an open work permit when you have an outland application in process but you do, at least, have the option to go home and work without affecting the processing of your PR. That's something you wouldn't be able to do if you had applied inland. Hopefully, once we know a bit more about your particular situation, we can give you some information that will help you know how much longer you might have to wait to have permanent status.