So to get my text field to not chew up the line breaks, I used property-set, and row-update in sequence. This seems to have done the trick!
In my PO Entry Line Details, I added a new container to make the line comments span the entire page. I left the original Epicor name in place for that text area.
Then in my onClick event, I have this:
Property set on component id PODetail.CommentText, with props: Prop=value, Value=“{PODetail.CommentText} {actionResult.OutText}”.
Row-update on epbinding PODetail.CommentText, with the Value “{PODetail.CommentText} {actionResult.OutText}”