The open source part (EDK/EDK2) does only contains very abstracted software infrastructure, arguably quite badly designed (e.g. you can see the hand of the psychopaths of MS putting GUID everywhere even and especially where it does not make any beginning of sense). I don't see why anybody would want to write programs for that environment.
What could be interesting to take for something which has the actual intent of booting a computer, like coreboot+its various payloads, instead of merely providing an ms-dos like os where linking is done by guid and the whole thing is burned down on your motherboard, is not open source at all. Depending on the cpu and chipset vendor, there are not even public datasheet for booting your chips.