I suppose your DH could always give his own gift "from DH" if that would make him feel better. Though the funds for it should come from his fun money, not shared funds, in that case (if applicable), because I think the videography you're doing is completely worthy of being labeled your gift to the couple. I think it's no more "not from him" than any gift where one person went to the trouble of picking out something and wrapping it, say, while the other person did little more than carry it from the car. And, if he's looking after the kids while you work on it, then he is definitely contributing!
I think the heartfelt card is a great idea now, and then you can "go the extra mile" with the video, whatever you feel that means--getting it done faster, adding in special features, making it accessible online, I have no idea. But something you're doing because this is your friend and you want it to be awesome. I think it's absolutely as much a gift as if you were making an afghan by hand or something; it's just that due to its nature, you physically can't have it done in time to bring to the wedding.
Maybe in the card you could say something about that, so the bride knows what to expect. I'm having flashbacks to the thread where the OP went from Australia to Vegas for a friend's wedding and was going to give them, after the fact, an album of wedding photos, but before she could do so she was upbraided by the bride for not giving a gift at all. I mean, hopefully this bride will not be that tacky, but saying/writing something like, "...and I'm so excited to make this video as my gift to you!" might be safer.