From what I can tell The OpenEmbedded project is already doing what you ask for:
Open Linux Palmtop OS Specification -, build it and maintain it (for all kind of PDA).
Developing for all kind of PDA is IMHO the Achilles heel of this project. New PDA models are coming out faster than the project finds (unpaid) enthusiasts who purchase the new device and start to find out about the specifics of the new device.
IMHO, the result is that there are stable releases only for older devices and as a user you must either try to eBay such an older device - or have the luck to buy the right model for which a stable software appears 1-2 years later. Or you have to be able (time, money, skills) to start porting yourself.
So, these projects should be focussed either on a low number of stable platforms (PDA brands/models) that are not easily abandoned by their manufacturer.
Or, an open hardware spec and someone willing to produce devices based on that spec is required. The benefit for them would be that they get a modern, free, easy to use Linux right from the net.
So, repeating myself - that leads to some form of a "Zaurus Foundation" (or "Open Handheld Foundation"). Amrein already has proposed what could be seen as a first draft of a "charter".
-- hns