Part-time, consultancy work in infosec?

Hi,

Imagine I am employed full-time (mostly working in systems planning and security implementations).

My schedule allows me to work up to 20h/week as a part-time consultant.

If I take a security certification such as the CISSP, do you think it is common to employers hire Infosec pros to work on a part-time, with flexible schedule ?
What is the hourly rate people are charging ? I recently got an offer to work on part-time basis but it seems CISSP was a required cert to have.