I don't remember having seen one, but a convolve is simple enough that it would make a good first project for someone interested in learning how to make Fuses.
Fusion's Custom Filter has all kinds of problems, anyway. You can type in decimals, but it just rounds them off to integers. If you switch to the Common Controls tab and back to the main tab, it will reset to a 3x3 matrix. And, as you pointed out, it's limited to a very small kernel.