There's nothing I saw the directly stated against it when I looked at Article 13 in the CBA, but it also doesn't say anything about a player being excluded from waivers outside the waiver period either. But then looking at the lockout where they had to advance that waiver period for players to be sent down (otherwise the wouldn't have been able to once the lockout started) it makes more sense that teams couldn't just exempt themselves from waivers by sending someone down early.
The source being a forum isn't that helpful for me, but then mine is the blogger basics as well (although supported by the CBA). As you say, you'd think we'd remember hearing about players that would be waiver eligible that were able to be sent down outside of the waiver period if that was possible.
I'm pretty sure that'd be a big loophole, but I sent a tweet to CapGeek and some others to try and clarify.
EDIT: as far as the players sent down earlier in September, all the ones I remember were junior eligible guys, guys on camp invites, and then maybe even some AHL eligible demotions. Last year's camp only started Sept 11 for physicals and fitness testing, and the season started Oct 1, so 20 days later.