There are two parts of the question that needs answering, as I see:
Am I obligated to pick up the maintenance of this code [....] ?
You answered yourself, "I inherited this code" (I take this as in professional capacity), so the responsibilities are yours, too. Whatever requirement comes that needs a change, you have to work on that.
[...] outside of project work?
No, you're not. As you mentioned, it was a "labor of love" to your ex-boss, is it the same to you? If the answer would have been yes, you would not have been asking this question here, so the answer is no, both the cases.
To clarify: "Maintenance" is not analogous to "change-the-code-in-my-free-time-at-will-because-I-can". It is a well-defined process, driven by a requirement (internal or external) and has a clear versioning of changes (preferably with a change-log).