I think you did exactly the right thing -- open a JIRA ticket, attach the code, and post here so people can look at it and offer feedback. Sometimes an SVN diff-based patch is a little better than a raw chunk of code, but for something as self-contained as this example, the PHP module is fine.
As it happens, I'm currently in the middle of refactoring all of the code dealing with MARC records (part of the long-discussed record driver project). As a result, the exact code that you changed isn't long for this world. However, I'll see what I can do to incorporate your ideas into the new version -- I do agree that it makes sense to favor ISBN-10 over ISBN-13 where possible since it tends to give better API results. I'll assign the JIRA ticket to myself as a reminder, and I'll let you know how it looks when it's all done.
Regarding the record driver project (see http://vufind.org/wiki/other_than_marc for more details), I'm currently doing all the work in an SVN branch, but it's still a messy work in progress at the moment. When it's a bit more stable, I'll post a link here so others can take a look.