So the optic would be partially on a solid rail (upper receiver) and partially on a not so solid rail (the RAS)? Um, it is just my opinion, but I probably wouldn't feel comfortable with that setup. Maybe mount it and see how sturdy you think it is going to be.
I've seen lots of other guys mount one scope ring on the upper and the forward ring on the free-floating handguard rail, but I just am not crazy about that idea either. If that forward rail were to come loose, it could stress the optic or mount. I actually had a DD rail come loose because I didn't snug up the handguard nut good enough. My fault for sure, but it can happen.