So, basicly you say, you made your own RS232 cable (sorry, my naming convention isn't soo good, but I asume you know what I mean) buy taking some piece off Cat5cable, and solder your own connetors, and it works?
It's even simpler than that. Instead of solder type connectors, use these:
and simply plug the cat5 patch cable's RJ45 into the adapter. The db9-rj45 adapter comes with 8 pins and an empty shell, so you can get the pinout you need by simply plugging pins into the appropriate holes in the connector.
If you were extremely worried about it you could use solder type connectors and make sure two different signals aren't using the same pair, or even use a pair as a single wire. I've never gone to that sort of trouble, though. I typically tie the pins one for one (pin 1 on the rj45 goes to pin 1 on the db9, etc) on one end. On the other end I do the same for a straight cable, cross 2 and 3 and wire 5 to 5 for a three wire null modem, etc. A full null modem with hardware handshaking can get a bit more complex and require a bit of soldering, but no worse that wiring the whole thing with solder cup connectors.
But I serious wonder about the shielding problem you will have, than again, as you said with low data-rates it should not realy mather. Think I could try it with some trial-and-error, hehe..
That has been my experience, but as they say, your mileage may vary. (: I have a production system (security cards/door locks) that uses a 50 ft cat5 patch cable and two db9-rj45 adapters for an rs232 cable. It's used many times a day for the past 5 or 6 years with no problems whatsoever. I think it runs 9600 baud.
I know that quality RS232 cable is usually shielded, but I'm not sure if the rs232 (EIA-232 to be more accurate) specifications call for shielding.
Good luck on your project!
-j