Of course it works. The filesystem knows which blocks the underlying storage device can forget and tell it using TRIM. That the content of the block was encrypted is irrelevant. TRIM is for putting blocks in some undefined state when no longer needed, not for zeroing them.
Oh it looks like I'm a bit out of date, it took years but I can see they now support this. Of course this comes at the cost of being less secure (giving away the used block locations), though it shouldn't matter for this use case.
See http://worldsmostsecret.blogspot.com/2012/04/how-to-activate...