Would it be easier...and more useful...instead of trying to hack into a j720 to find a way to add a second pcmcia slot - externally? Or hack to put an internal usb port, internally converting the serial to usb 1.x?
I haven't a clue why you'd want an external PCMCIA slot when you already have an internal one.
And the work to add USB support would be "fun" to say the least.
Another possibility might be a CF-to-PCMCIA adapter, if you're willing to give up the CF slot at least some of the time. (I got one recently, and it's not nearly as cumbersome as I thought it would be.)