Connecting to the parallel box should be no problem. I was out of town last week and all my parts I ordered came in so I was able to do a quick test this weekend.
I soldered up a dreamcast controller and connected the wires to the swticher. I used Printer cables and it didn't worked. I don't know why, but here is my theory.
When you strip open a parallel printer cable, there are about 20 stranded cables on the inside. I mapped out these wires to there respective pin by using a continuity tester. All the printer cables I striped had the same color scheme so I assumed that the wires would map to the same pin. I was wrong, this isn't the case! For example, I had on cable where the brown wire mapped to pin 5. On a bother cable, the brown wire mapped to pin 15.
So I wired up my on db25 connectors. I was in a hurry so I just did button A on the DC controller. I set up another db25 connector and hooked a button up to it. Connected the db25's to the switch box, put in a copy of Street Fighter X, and it worked! Ryu was kicking everytime I pushed the button.
Conclusion, if people were having problems with the parallel cable switchers, I don't think it was due to the switches, I think it was because of the cables! I will be soldering my own db25 connectors instead of mapping 8 different sets of printer cables!