Does anyone know of a feat, or trait, to allow a multi-classed cleric to improve his effective class level, re domain abilities?
I'm thinking in terms of Practiced Spellcaster, from 3.5 (Complete Arcane), which did the same for caster level.
Also, can someone confirm if the bonus domain spells/day, and domain spell list, are capped at cleric level, or if they increase with caster level from other sources?
If they are capped, I'd like a way for them to also improve.
They are capped by cleric level, not caster level.
For example, a cleric 8/Holy Vindicator 4 would cast spells as an 11th level cleric, but would only have domain powers as an 8th level cleric. (He would however get his extra domain slots for his higher level spells and could cast them per normal.) Due to the nature of the Holy Vindicator class, he would also be able to channel as a 12th lvl cleric; most PRCs wouldn't give this benefit either.
I'm not familiar with a way to increase ones effective cleric level for purposes of domain abilities. Although i think adding multiple classes that have access to the same domain would do so, such as adding levels of Inquisitor to levels of Cleric (but only for the one domain that the Inquisitor has, not for both domains.) However, this doesn't add any caster levels to the cleric.
One other option would be to take a race that offers a favored class benefit for clerics that grants more uses of a domain power. For example, a Dwarf cleric can use his favored class bonus to get more uses of his 1st lvl domain power. But he can only do this when levelling as a cleric; once hes in the prestige class he can't add more.
It may not be quite what you are asking but, if you are interested in lots of ways for a cleric to improve his domain abilities you might want to check out the Exalted Domains series from Super Genius Games:
The Genius Guide to Exalted Domains of War and Ruin
The Genius Guide to Exalted Domains of Light and Lore
The Genius Guide to Exalted Domains of Storms and Savagery
I have it on very good authority that they are pretty cool :)