Look at what has changed with Maemo5 that can't be run on N810. While some software could still be used on N810, HW features of N900 are in use in many applications. You would not have a proper camera application with appropriate image quality, as much of work is split between both HW and software implementations. In order to support 640x480 camera in N810, a full software image processing stack would be needed but end result would not be good anyway as front-end camera is simply not the one for photographic purposes.

There are many examples like that, including many optimizations for performance of multimedia which are not applicable to N810 hardware (Neon-specific).

As very early conclusion I would say in case some new functionality is dependant on HW features, it is almost impossible to reliably allow it supported on previous products. If such a "past generation device" project would be created, it essentially means split in both functionality and UI experience, i.e. doubled effort.

Now, real question is how to scope proper content that can be supported on "past generation device"? This exercise as an input into effort estimation is probably worth to perform.