Probably a dumb question, but is there a dreamcast/xbox/gamecube/ps1 equivalent of this (use your old game discs and upscale them with the emulator)?
For that matter, do Ps1 discs work with PCSX2 since the Ps2 itself was backwards compatible?
No, PCSX2 is strictly a PS2 emulator. While the PS2 is backwards compatible, it's partly because it has unique components (IE PS1 hardware) in the PS2 itself. While the PS2 makes SOME use of the PS1 hardware, there are unique parts that just aren't emulated.
On top of that, the devs aren't exactly sure HOW the PS2 switches into PS1 mode and aren't too eager to dig through the bios/the memory of a running PS2 to figure it out when there are dedicated PS1 emus that would run it better anyways. And merging a working PS1 emu into PCSX2 that'll "flip" like a switch just to say "it plays PS1 games just like a PS2" would be both incorrect and silly.
THAT said, GSDX (the primary graphics plugin for PCSX2) is 100% compatible with PS1 emus that use the PSPro plugin spec (IE almost all of them), and it DOES work for rendering PS1 graphics. So if they ever get to that point, the graphics plugin at least will be ready to go