I've used a class of products called Terminal Servers to connect as many as 64 serial ports to my computer...using a single CAT-5 cable. I've only used products from DIGI, but Lantronics and others offer similar things. Of their product offering, I've only used their least sophisticated models.
You simply connect the Terminal Server to the LAN, give it an IP address, load their software and tell it what the IP address is, and suddenly you get lots of serial ports to play with. The Terminal Server could be located close by or on the other side of the plant for all you care.
The only warning I know of is that not all software programs can handle port numbers greater than 8 or 9. VB doesn't (or didn't 2 years ago). The KERNEL32.dll will accept the following syntax:
"\\.\COM" & portnumber
For example, if the variable portnumber = "64", the string becomes:
This trick alone is worth the price of admission.
Poor planning on your part does NOT constitute an emergency on my part!