I was shopping for a simlar thing on the weekend.
My newer PC has only a ExpressCard slot, so I was looking to get an adapter to plug my PCMK in through the Express slot.
I think this solution may work, as they are both extensions of the PCI bus, but the USB-PCMCIA? - I doubt RSLinx would know where to look for it. The driver is looking for it to be in a certain area of IO address.
I am also going to keep my two old laptops around for the purpose of having PCMCIA and a real serial port.