I found during my search was that there are LOT of "manufacturers" of these MP4 players. I put "manufacturers" in quotes because they're not really manufacturers, more like assembly plants. What they do is buy parts like chipset boards, screens, memory etc and assemble them/put them together and label them as their own or leave them as unbranded.
So the biggest problem is getting a hold of the chip manufacturer that will customise the player for you. Some manufacturers can do it, but it's just a matter of paying the chipset manufacturer to have it done really.
I found that doing business over the internet is really difficult, especially in China. If you really want things like customised and access to the firmware etc, you are better off going to China and meeting with them yourself. That would be the best bet and you know what you're getting. You won't want to be rip off as well.
Also, I've been told that a lot of these "manufacturers" close shop OFTEN! A lot of them close each year, but more 'pop up' in that year too... Wonder if there is a pattern....?