How to forward domains with Multisite installed?

Okay, so I have several clients that have 2 domains. In many cases I need them both as parked domains in cPanel because their "old" domain has email addresses assigned to it. So I add them as parked domains so I can setup email addresses so they continue receiving email from those addresses.

However, in several cases we've also put their site at a new domain using the WPMUdev domain mapping plugin. Now I need the old domain to forward/redirect to the new domain. I tried using cPanel's redirect feature, but it doesn't work with Multisite and wildcard subdomains. It just redirects to the main site's registration page.

How can I redirect these domains, while still keeping them as parked in cPanel, and handling email accounts? I just need it to (preferably) 301 redirect to the new domain. Any ideas?