Well, ... no.
Weight can't be replaced by anything other than even more weight, so if you want a heavy keyboard, you'll have to add something heavy to it. The Model M is heavy due to the absolutely over-the-top steel plate (even computer cases use thinner steel), but something like this is simply not required with PCB-mounted (or even plate-mounted) mechanical switches. Your only option will be to add something heavy (platinum, gold, tungsten, uranium, tantalum, mercury (though this is not recommended for obvious reasons), lead, silver, copper etc.--steel/iron is actually not THAT dense, but it's readily available and cheaper than other options, so you might want to go for that) for the sake of weighting it down.
-huha