I think that what I may do is get a USB to RS232 cable, open it up near the RS232 end, and splice out the USB +5v and ground wires to a JST connector
but these cheap USB to RS232 cables are about $3 shipped.
Is the idea here to create a connection from each computer to the unlock mechanism? If so and the computers are on a network, you may consider an Ethernet shield or computer with web server.
The main reason I haven't just bought an FTDI cable for this is my budget. I will be donating a couple of these to my colleagues in my lab at school and I am currently on a grad student's income. The cheapest FTDI cables I have seen are about $15, but these cheap USB to RS232 cables are about $3 shipped.