Custom patches - is RGB info possible over 1 or 2 dmx addresses?

If you patch them as Generic Hue/Sat fixtures and then have Arduino do the conversion that could work.

you can use HSI, by default htey also need 3 addresses, hue, sat and intensity. you can get away with hue and sat only but then you have no way of turning off the pixels. R=0, G=0, B=0 equal "black", but H=0 and S=0 equal white (and no, bring one or both to 255 doesn't turn off the lights).
you can get away with two addresses per pixel (and some custom code): one for color and one for intensity.

How would you make a custom parameter for colour? Colour presets and macros seem to be an option in customising a profile, but I can't see how to actually get info into it.

i would use the parameter name Color_Select and then assign a color wheel to a channel. if you patch a regular moving light with a color wheel and go to Patch>Attributes, have a look at the Color_Select field in the CIA to see how that's done. the custom code would be knowing which DMX level represents which RGB values for the pixels output. not sure if pixel mapping will control the "color wheel" though, i have a feeling it will not.

You could take a different route, though not as much mapping via the console if that's the exercise you are trying to accomplish.

Build some prebuilt animations, and store them on the Arduino (or another networked device). Then playback animations similar to how virtual layers do in the pixel map servers.

Pick your animation with a DMX channel, dimming channel, playback speed channel, etc.  (could have a really fun setup with only a few channels of operation.)

Here is a related video you might find interesting with or without this technique:

https://www.youtube.com/watch?v=TvlpIojusBE