Well recently someone published an UART implementation:
I couldn't test it though, but it seems pretty straight forward.
However, I'd like that sending DMX happens in background and I think that's not the case. Besides that there is another problem and that's that DMX needs a BREAK (>88us LOW) and a MARK (>8us HIGH). So you have to change the UART to GPIO for a while and back (like the implementation in particle-simple-dmx). Because of that I suppose that not everything can happen in the background).
That's why I like the array approach that I found online and I tried to modify that for the GPIO. However because I couldn't make that happen I've tried the SPI road. Which seems a good suggestion, except that you can't set the timing precise.
I've put the code in a repo:
https://github.com/kasperkamperman/PhotonDMADMX
I tried to copy the DMA setup from the SPI3 code in the Particle Firmware, however I think it goes wrong with the connection of the timer.