In my original question, @DG, I was looking for 1 column of text on one side of the page and then lines for the other half of the page. Since PTXprint only gives control for a "side margin," having a 1-column layout requires that such a column be centered. @DG, from your answer, it seems like you may have the same view on that.
My solution, then (as stated above), gives three columns: two sections of lines on the sides (delivered via the background PDF) with a column of text in the middle (formatted by PTXprint).
As a follow up, if the lines-to-text ratio in such a layout seems too high, it is also possible to get 2 columns of text with a column of lines in the middle. To do this, I maxed-out the gutter on a 2-column layout. [By the way, the UI limits the user to a max of 40mm, and then it only gave me roughly 31mm in the PDF.] Then, I can create a custom PDF that gives lines in the center, and the PTXprint configuration gives me a 2-column layout that straddles a column of lines that are about double the width of a normal center-column cross reference area. This gives much more of a normal look, which may or may not be desired for our Scripture Engagement purposes.
Perhaps the 1-column-of-text layout could be used for teams/groups that want to have more room to write, and the 2-column configuration could be used for groups that are more cost-conscious and won't want to be doing as much writing.
PS. @Mark P, regarding the headers, we want this to look different from regularly printed Scripture texts, so a centered reference range seems to do the trick for either 1-column or 2-column layouts.