Hi, spdaneels
I am not very sure if this solution can solve your problem, because your requirement is not very clear in your description.
http://www.sharepointboost.com/alerttutorial.html
It can set sending conditions and include email addresses as sending targets. I think your custom field type may be included into the sending conditions, and I hope it can help you!
www.SharePointBoost.com
SharePoint Add-ons: Enhancement for Alert, Reminder, Permission, Lookup, RichText, Password change,List Collection, List Form Validation, Lookup Boost
services@sharepointboost.com