While it's impossible to say definitively (someone could go make a keyboard out of lead tomorrow), I have never heard of anything like this and would say it's both impractical and not really useful for the reasons stated below:
Keyboard mechanisms are designed within very specific tolerances. In order for escapement to work properly and be useful, the key has to be balanced so that it returns very quickly from any pressure exerted. These mechanisms will vary from manufacturer to manufacturer (so you may find one make of piano has a slightly more weighty action), but the concept of weighting a key unnecessarily is throwing everything out of balance; the added inertia will prevent the key from returning quickly after a press.
You're essentially talking about strength training for your fingers. While this is fine in a general sense, practicing piano technique on a keyboard with drastically different feel from the standard practice is going to put your own perception completely out of whack. You'd likely be very good at playing everything loudly, but you'd have to retrain yourself on a standard keyboard to learn to play soft. Given how subtle piano expression can be, that's not something I'd want to put myself through.
Like filzilla said, you'll get much more over the long term out of specific finger-strength-building exercises played on a standard piano. I too have weakness problems, particularly with my ring and pinky finger, and have found Hanon Part 1 #s 4, 7, 9 to be particularly relevant.