Are you talking about a "baybus" type device that controls more than one fan, or are you talking about just a single rheostat for a single fan?
I have seen a few fans with a speed control mounted onto a PCI slot cover.
Most probably, you can buy a rheostat type kit, drill an appropriate-sized hole in a PCI cover and just mount the rheostat in there. Lengthening the wires, if need be, is an easy enough process.
I have never seen a multiple-device controller made for a single PCI slot cover space.