Hello i think im also giving up on the TinyCore builder. the site IS back up but i think with the ubuntu its gonna run better and plus USB redirection and all that seems to be working.
I will follow step by steps you have posted SebMig and of course ebelange. Going to start from existing 12.04 ubuntu server(serving as DHCP and PXE boot server) debootstraping from there and doing all the steps SebMig posted. I hope to get it working. And to get some help if needed 
For info - our usage scenario is : repurposing 4 - 6 year old pcs in our students lab to be thin clients. Like many of you here i dont want to deal with managing those thin clients. just wanna power on the machine and pxe boot it. What i was afraid of is that the ubuntu pxe boot will take as much time as windows pxe. but it seems that you are reporting boot ups in less than 50 sec. So this all should work perfectly for us as long as viewclient is 90% as good working as it is on a windows enviroment.