Ok, did that, forge on 1.6.2 works. Now for single player commands. Crashed because of a binary discrepancy similar to the previous one, but with net.minecraft.entity.EntityPlayerMP. On the SPC forum thread, they did say it was compatible with Forge. I am now more confused than I was before.
I used the installer, and I crash because of a binary discrepancy error between the expected input class net.minecraft.client.Minecraft and the actual class (quoting the console), and I did not edit the jar beyond using the installer. EDIT: Just did some jar fishing, that class doesn't exist.