Hi Dan,
I can think of a few options around the OLA/open source end of things. For the dawn/dusk, a quick Google found this, I've never used it, and I'm sure there are lots of similar things, most decent programme libraries with a good selection of contributed libraries will have some means of doing dawn/dusk calculations, it looks like this has implemented it and done most of the leg work:
You could possibly do some stuff similar to tracking by running two ola_recorder instances and using OLAs merging algorithms to deal with them, if it's just as simple as not turning off HIDs, although I appreciate that won't give full tracking, but maybe just controlling different fixtures via different recorders might be sufficient?
You could also look at other software such as QLC+, either completely standalone, or outputting to OLA so your DMX output side doesn't need to change at all. I'm not sure whether it does tracking, but I think it's one of the most mature, or at least longest lasting open source lighting consoles.
You could also roll something yourself with our APIs if you really wanted.