Buffer alpha + matte issue

[ingreme]

I'm new to Octane, been working with it for 2 weeks now. Since we work with compositors i'm having some struggle with some things.
How do you check the current image alpha on octane viewport?? Such a basic and necessary thing, and apparently it doesn't have that feature?

Also, if i want to do several white mattes of different objects, how would i go about this? We're used to Vray, and it's really easy to do there, and it's such a useful feature for compositing, specially our workflow that doesn't incorporate multimatte elements.
I just want a pass with X objects on white, rest on black, another with Y objects on white, rest on black, etc.

Thanks