I agree with those posters who say that although announcements are perfectly correct and do not obligate the recipient to send a gift, there could be many recipients who nevertheless do see them as a bid for gifts. And even those who do know this may wonder, because announcements are so rare now. I cannot remember the last time I got one -- many, many years. It doesn't matter what the rule is if what you are concerned about is not only whether you are technically correct but also about how they will feel and how you will look.
sammycat's observation hit it on the head for me:
If I'm not close enough to the HC to be invited to the wedding, or to even hear about it on the grapevine, then I'm probably going to be puzzled as to why I'm receiving an announcement, and likely won't care anyway.
As others have noted, there are so many ways now in which people get news about each other, there isn't the necessity today that perhaps there once was for mailed, formal announcements. You can easily tell the handful who wouldn't otherwise hear some other way. So I think that if I received one, I might wonder if it was a prompt for a gift or some sort of extra fuss, for exactly the reasons that sammycat said.
I might feel different if the couple had eloped or married on the fly with just immediate family. In that case, I can kind of see the couple, or maybe their parents, wanting to get a little bit of that community & family wedding feeling, not necessarily even thinking about gifts, and this would be one way to do it. But a 50 person wedding would do that, so in this case I'm afraid the effect might be like Ceallach describes:
To me it comes across almost like rubbing people's faces in it e.g. "Look we got married! You didn't make the cut though!"
But I certainly don't think there is a problem with your letting people know. I agree with the posters who said to do it in other correspondence, however you usually communicate with people. As you frequently send cards and letters, that would be ideal. For anyone who isn't getting one any time soon, you could call, write a note, or email, however you usually communicate with them. Some of them would surely have heard already from other relatives and such anyway.
If you really, really want to send out announcements, I suppose you could add "Please no gifts." Of course it is incorrect, but it would in my opinion neutralize any "Is this to try to get me to send a gift?" effect.