RGB cable for PS2 will also work with a PS1... I have one cable for both machines! Same power cable too, so you can technically place the PS1 on top of a Phat PS2 and hot swap very easily.
The thread on PS2 disc lenses dying has made me finally want to go down the HDLoader router. I have a phat PS2 (Canadian launch model) whose laser is practically dead. I also have a Free McBoot memory card, and a network adapter.
To me, it looks like the easiest hook up would be IDE 40 pin to 44 pin adapter + 44 pin to SD adapter. I could easily tuck both of those into the HDD bay, and run games from the SD card. I've seen a few mentions of this working on other forums, anyone have any experience using this method?
I would rather suggest a PATA to SATA adapter and using a 1 tb 2.5" HDD