I feel like this might fall more under the territory of "DM Call", but I'm interested in what peoples opinions are on this interaction:
The Stars Are Right (Su)
If you prepare your cleric spells while the stars are visible to you, you may spontaneously cast any of your Stars subdomain spells by swapping out a spell of an equal spell level. Any Stars subdomain spell that you cast while the stars are visible to you heals you of an amount of hit point damage equal to the spell's level; this effect happens as you cast the spell.
Domain Mastery
At 1st level, when an ecclesitheurge chooses his cleric domains, he designates one as his primary domain and the other as his secondary domain. An ecclesitheurge can use his non-domain spell slots to prepare spells from his primary domain's spell list.
Each day when he prepares spells, an ecclesitheurge can select a different domain granted by his deity to gain access to that domain's spell list instead of his secondary domain spell list. He does not lose access to his actual secondary domain's granted powers or gain access to the other domain's granted powers. For example, an ecclesitheurge with Glory as his primary domain and Good as his secondary domain can choose to gain access to the Healing domain; until the next time he prepares spells, he uses the Healing domain spell list as his secondary domain spell list instead of the Good domain spell list, but still keeps the granted powers of the Good domain and does not gain the granted powers of the Healing domain.
This ability alters the normal domain ability.
~~~~~
If I took the stars subdomain as my secondary domain, would I be able to swap out the spell list with domain mastery but still cast it spontaneously with the Stars domain ability?
Mechanically speaking it would make sense, you're still using the Stars domain, you're just replacing it's spell list.
You prepare your spells under the stars. This allows spontaneous Stars domain spell casting. You swap the Stars domain spells on your actual spells prepared list for the (let's say) Healing domain. Your domain spell slots have the Healing domain's spells. You can give up any of your spell slots for an equal level Stars domain spell.
Sounds like it works to me.
Yes, you'd be able to use Stars Are Right to spontaneously cast Stars spells.