I don't see how. If one saw the Maker as existing, but as less hostile to other faiths than the Chantry sees him, I don't see how it'd be incompatible to revere both the Maker and the Creators, for instance.
I disagree. We only know the Maker through what the Chant says about Him. And the Chant says this:
"As there is but one world,
One life, one death, there is
But one god, and He is our Maker.
They are sinners, who have given their love
To false gods."
The theologies of the Elven Pantheon and the Chantry are incompatible. This verse proves it. The only way to combine them is to selectively choose which parts of their selective theologies you think are true, and which ones you believe are false.
But once you start doing that, do you really believe in either of them?
It's like saying you believe in God, but not the Bible. The Bible is how the world knows about God. Once you throw it out, you've created a new god with the same name and some similarities with the god that Bible-thumpers worship. But this God is not the same. He has no set boundaries and words by which we can know Him. He exists on the whim of those who worship him, and ceases to be a god at all, eventually.
*End off-topic tangent*
Edit: To make this post on topic: Cassandra believes in the Maker, and the Chant. I highly doubt you can bring her to accept any other worldview than that. She may accept it if your Inquisitor holds another view, but it's going to be a serious barrier to romance, IMO.
Notice how, hardened or not, Leliana is still a believer in the Maker? Cassandra served as the right hand of the Divine for 20 years. A few conversations with the Inquisitor are not going to shift her faith very far. Change aspects of it, sure. but you're not going to make her an atheist. This is my opinion, based on all the people I know who are of a deep religious faith, and intimate knowledge of those I have seen fall away from it. Those who leave their faith(not just their church, but their faith) behind either never had strong faith to begin with, or let their faith wither to the point where it was very hollow.
Cassandra doesn't strike me as the person to do either. So romancing her as an atheist Inquisitor, to me, seems like romancing Isabela as a celibate Hawke. It's going to cause issues.