Can I use a number @2 aluminum ser 4 wires for a 100 amp sub panel?

I do.
Some areas do not allow this, many do. If your area does not allow it you can use it and use a 90A breaker. You'll NEVER trip it anyway.

I agree with Stan----using T.310.15(B)(6) should be fine based on your install. Yet, it doesn't hurt to check with your local AHJ (Example of AHJ: Electrical inspector).