Certainly-- that was somewhat sloppy of me. In my defense, however, a priori and conceivability/imaginability are pretty inextricably tied. Additionally, you yourself used the word "envision."
your brain will only be able to envision...
It would perhaps be helpful if you could clarify what you meant when you said:
If you can't come up with a good answer to that, it's not observation that's ruling out "non-reductionist" beliefs, but a priori logical incoherence.
Your usage doesn't seem to fit into the Kantian sense of the term-- the unity of my experience of the world is not conditioned by everything being reducible. What do you mean when you say irreducibility is a priori logically incoherent?
See blog post links in Priors. A priori incoherent means that you don't need data about the world to come to a conclusion (i.e. in this case the statement is logically false).
Here's our place to discuss Less Wrong topics that have not appeared in recent posts. Have fun building smaller brains inside of your brains (or not, as you please).