OK, I may have found just the ticket:
http://www.gudensoft.com/emailscheduler/
"Email Scheduler is a simple service program for Windows Mobile Devices that will automatically change your email polling settings. This means that just like ActiveSync, you can now have a schedule for all of your email accounts to check for messages. Setup is a snap, just install the cab, run the email scheduler settings program, set it the way you want, and save! The rest will be handled by the program. This is actually 2 programs, one is the service that gets launched when needed, the other control's the settings. WM can launch programs at certain times, and this takes advantage of that so there is no constantly running program (no footprint!). "
I just installed it, looks promising.