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I've discovered some interesting bits...
I put code into GypPlay to allow the swapping in of real GD-ROMs. I then discovered that some DC modchips are doing some interesting things which trick the DC into believing that a CD or CDR is actually a real GD-ROM. Therefore, people with those modchips are actually able to swap in CDRs and GypPlay will play videos off of them without the disc actually having GypPlay on it!
First, to tell if a modded DC will support this, power on the DC without a disc in and let it go to the main menu. Then, put in a REAL, IMPORT GD-ROM (which is what your modchip is for, of course). If you can then just go to play and start the import, then your modchip will likely allow CD/CDR swapping from within GypPlay. If your DC says that the disc is invalid or just says "please insert game disc", then your modchip only supports playing imports at boot (instead of from the main menu) and will therefore not allow disc swapping from within GypPlay.
If you have determined that your modchip does allow swapping, then boot your GypPlay disc and start a movie playing off of that disc. Once the movie starts, open the DC disc door. GypPlay will return to the main menu. Insert your CD/CDR with additional movies on it and close the door. Let it spin up and then press A to start. GypPlay will begin searching that disc and will play the videos found on it.
Note that this only works with MODE 1 (not MODE 2/XA) discs. Therefore, inserting an original VCD will likely not work because it is mode 2. However, inserting a CDR of a VCD (likely mode 1) will work just fine and GypPlay should find and play the DAT file on the VCD.
I personally do NOT have a modchip that allows this, but I have confirmed this with three different people with two different modchips (two 5-wire and one 4-wire) that got this to work.