system_partition_table_regist();

Does this change in V3 where the system partitions need to be defined at build time mean that the target device flash size etc will be hard-coded into the firmware during build with the result that you won't be able to flash devices with different flash sizes with the same firmware as you can now ?

Does this change in V3 where the system partitions need to be defined at build time mean that the target device flash size etc will be hard-coded into the firmware during build

No, it should be possible to dynamically fill in the structure that is passed to system_partition_table_regist() and so configure the flash size on the fly.

Closing due to disinterest. If anyone is interested in getting this working, feel free to test it and provide feedback.