Subtheming to add a single template?
The theme settings not being inherited by the subtheme is a Drupal core bug. If you simply copy the theme settings from the parent's .info file to the subtheme's .info file, it will alleviate the issue.

However, keeping a local patch of the theme is a reasonable solution in this case. The rule of "dont edit a contrib theme" is really in place just to ease potential updates, etc. for users who may be unaware of the risk of losing their changes upon updating themes/modules.