Well, directly editing the .rcp file using Notepad++ works quite fine. Go to the Language menu and select JSON to help so you don't inadvertently make a syntax mistake.
Thanks to @brentp and the Autosport Labs team, the variable names are obvious enough to support manual editing if you want to copy and paste and alter to make a new CAN signal.
So I'd say this is a pretty low priority feature request. I'd much rather have the bug in bitmode mask get fixed instead.
