Is this the project you are thinking of? http://www.ltsp.org/
It seems to be geared more towards thin clients, but it might work in this instance. It would require heavy tweaking though. You mention VNC again, but you have to remember I'm new to this. I don't know what other distributions are available on CentOS or linux in general. Did you have one in mind? As for the spice make problems, I've not gotten an answer from the spice-devel list. It looks like spice is not going to be the solution I need.