I have an idea of implementing rather cheap solution for kvm. During research of how implement it I found www.opengear.com and link to your project. So if you can provide feedback about this idea that would be priceless. Or you can use it on your project if you like.
So I looked around and did not find solution where pci card for kvm emulates video card by itself. But such implementation can make hardware very simple - you can just send framebuffer context over the wire. For text mode it makes traffic very low 80x24x2 bytes per screen and using simple algorithm to send only changes makes it even smaller. And if you have unix/linux box that you need to monitor it is more than enough to have text mode only. It could be valuable to use the same solution even to simulate graphic mode through generic VGA or providing special driver but I do not have enough experience with hardware development to measure.
I have looked through on how to implement such hardware and see three ways:
1 Get existing VGA card design and attach microcontoller with ethernet to its memory interface. If it is possible/simple or how it can detect video mode change is questionable.
2 Use pci controller like Cypress CY7C09449PV-AC and attach microcontroller with ethernet to it. It could be simplier than 1 because controller itself will simulate video mode change etc.
3 Use microcontroller/microprocessor that has PCI and Ethernet interface aboard. I found two with it NetSilicon NS9750 and Freescale (Motorola) ColdFire MCF547x. Cost of both is around $18 and it seems that they need only RAM, ROM and ethernet PHY to make complete product.