[...] if the firmware can be changed, we want an open source version of that too! we also want to be able to run our own code on it. signed firmware is a hostile statement saying that you don't want anyone else to be able to write firmware for this card.
The signed firmware is not intended to interfere with the consumer / user, in fact one of the key justifications is continuing to provide post-manufacture updates of the video card firmware to provide fixes and enhancements, while preventing counterfeiting where low-end cards are re-flashed with bogus firmware that factory overclock it and reports itself as a more capable higher-end (more expensive) model. Nvidia claims to have found unauthorized manufacturers / re-packagers selling such cards in Asia.
Users / non-Nvidia developers do not have the necessary technical documentation to produce their own firmware, so there is no lost of functionality or flexibility. Their video cards designs, ASIC, and firmware are all proprietary design, with almost no technical documentation available to open source developers.
I'm not aware of any cases of independent edited / patched binary firmware for video cards, since they all provide an API to adjust the clock frequencies via their host drivers. There is no need to adjust clock frequencies (i.e. end-user overclocking) via firmware.