Does Supermicro IPMI require OnBoard video to be enabled?
Yes, it does.
The IPMI controller and video controller are linked, which is what allows the IPMI controller to actually get you the contents of your screen. When you are using an external display adapter, all the "rendering" takes place on the card, so the IPMI adapter has no idea is being displayed.
You could plug in an external KVM unit from eg. Lantronix that would work with your PCI video, but I wonder how you need both PCI video and remote access.
Yes the IPMI can only display via the graphics card that is integrated with the IPMI controller, on a SuperMicro this is usually a Matrox or an AST, as listed in the specs of the motherboard.
This can get especially interesting if you use a newer Intel CPU with integrated video, like the E3-1275 v5. The IPMI can only display video via the AST video card, not the onboard Intel card. If you are not using a C226 or C236 chipset, the integrated video on the CPU is likely disabled entirely.