Your references (1,2) were extremely helpful in understanding what exactly is going on; Thank You!
The ’A few words about table expressions and op…' section in Introducing Arquero really explains the issue.
Unfortunately, for my use case, I’m dynamically creating new columns/attributes for the rollup-table and can’t reference columns/attributes that are being generated in the rollup as parameters for the op.corr method.
The params method/pattern is only applicable for calculating values as a function of the current table row being processed. It is a similar scenario to what @bmschmidt explained in this notebook.
So my use of the eval appears to be the only option so far for my specific situation.