Would be acceptable for a dawnish hause to use a sun in their heraldry?
Yup. Off the top of my head Houses de Soleil and d’Acier both have a sun in their heraldry; there may well be others I’ve not seen. Personally I’d avoid using a sun on its own and using a blue field to avoid confusion with the Dawnish flag, but beyond that, go wild.
The sun-on-blue is the Dawnish national symbol, and is similar to a couple of specific Scottish tartans: Released to be worn by all proud members of the nation.
However it’s also a heraldic symbol. I don’t recall what it means at the moment, beyond (national) Pride, glorious ascendancy, and life-giving power, but those are all good
You could think of it as similar to the Union flag in the flags of the Commonwealth: A statement of affiliation and pride, but different as an element of a heraldry than as the main symbol.
D’Acier, incidentally, had tower against the sun not just because they were vigilant (tower) on the eastern borders of their nation (Sun of Dawn), but because if you looked east in any of their lands in the summer mornings, that’s what you saw: Their castle (Sunrise Keep) against the rising sun.
There is a list of current Dawnish noble houses and their heraldry here
So pretty much the main thing to avoid is your house being just seen as knight errants, I would probably avoid blue as the field entirely to be safe. Other than that go for it
Interestingly the Sun and Moon are pretty much magically inert, (aside from a role in Urizeni hearth magic) So in Dawnish heraldry it is likely just a symbol of the nation.
I mean, the moon being made of ilium is a thing I’ve heard being thrown around a lot. Who knows for sure if the moon is magical or not?
It might be made of magic but it is not magically resonant.
Specifically the Sun has no major resonance with day magic, despite what a layperson might believe.
For the Urizen the sun does have magical implications (hearth magic) as it creates light and is seen as an “eye of the heavens”.
From the wiki
Events performed when the sun is clearly visible in the sky are “witnessed” by the sun
Unless your Urizen though, its just a hot ball of gas.