Doing a little research, I came across Sanford's suggestion for a .js & css solution to toggle a submit button, based on a select field.
It seems to be a pretty straightforward drop-in, but now I need to figure out what's preventing it from working for me. I just discovered one difference between his example and my form sitting on a Marketo Landing page, that might be causing some of the problem. In his example, the submit button's "mktoButtonRow" is sitting inside a "mktoFormCol" field. Mine is not inside the mktoFormCol, it seems. I'm wondering what the difference might be. Maybe there's a setting somewhere I need to change - or there's just something about the script I didn't understand and need to change.
For reference... his example: MktoForms2 :: Conditionally Show Submit - JSFiddle
And I went ahead and activated my test page for a moment to show what I'm trying: http://go.cyberscout.com/CyberScoutcom-Form_testingform.html