I'm sure you are aware that the CBP will accept the following documents for travel to the US:
Canadian passport
Enhanced Driver's License/Enhanced Identification Card
NEXUS, FAST/EXPRES
SENTRI enrollment cards.
I have been told by a US passport agent that the best (and apparently legal) substitute for a passport is a color photocopy of the original passport, accompanied by an original photo (not photocopy) identical to the one in the passport. Most people probably don't have an extra photo from when they applied for their passport, but it's worth getting an extra photo next time.
There is no way to guarantee that they will make any exceptions for you if you arrive without one of those documents. Sometimes it has worked with a birth certificate and driver's license, but it is completely at the discretion of the border agent. If you choose to assume the risks and try, make sure you have the following additional documents:
- Proof that you will return to Canada - VERY IMPORTANT
- Document(s) that show why MTL kept your passport (ex. interview letter, DS-5535, etc.) The consulate will not give any other document for the sake of traveling, and will tell you not to travel if you ask.
Good luck with your wait. "A few weeks" in consular language stretches pretty long sometimes. I've been there.