You are right, you cannot directly do this via workflow.
Rather than resort to a trigger though, can I suggest the following approach.
Use a Workflow rule on Case Comment to enact a field update on the parent Case, which (ensuring you re-evaluate workflow afterwards) can then fire a Workflow rule on the Case (due to the update) that sends an e-mail out to the Case Owner and your custom field?!
Sounds complicated, but it's not, I promise.
If that "solution" is enough for you to follow through, then perfect, if not you can read/follow this blog post which I wrote to set a custom template, which uses this exact Workflow pair to send the Case owner an e-mail on Case Comment creation - just ignore the custom template bit and follow the additional field work and workflows to trigger the e-mail.
You need the new field and updates etc. on the Case so you only fire the "updated" workflow when it is a new Case Comment causing the update, and not just a user editing a case.