Cover letter: Include a reference to an extracurricular?

I am toying with including a reference to my (non-career related) involvement with Amateur Radio (digital communications) in a cover letter.

I know the generally strict rule is to keep it out of the resume (and I have) AND I'm wondering if it might be appropriate here in the cover letter only.

My professional background is Telecommunications Engineering. My career has been in wired and optical networks. This job is more in the digital radio networks sphere but is still IP networks. So my extracurricular is closely related but not technically classifiable as professional experience.

Should I reference it in my cover letter?

Opinions please.