oh boy, this is a big "DUH" for me!
the 2nd persistence.xml was from my ejb jar (a dependency) -- I left it in there accidentally. Once I removed it, Run As - > Run On Server worked beautifully.
Maven must have been using an "installed" version of the ejb jar that did not have persistence.xml in it and that's why it looked "correct" to me.
Somehow no matter how long I linger before I press teh "Submit" button when I write one of these up, the answer becomes obvious only AFTER I've pressed it! Usually within 5 minutes of pressing it, too.
Have a nice day.
TDR