R – How to hide a custom field type from new column choices

Include a content editor web part in the page (newform.aspx / editform.aspx) and use jQuery (or just plain javascript) to handle the setting of default values.

Edit: some example code:

In the lists newform.aspx, include a reference to jquery. If you look at the html code, you can see that each input tag gets an id based on the field's GUID, and a title that's set to the fields display name.

now, using jquery we can get at these fields using the jQuery selector like this:

By title:

$("input[title='DISPLAYNAMEOFFIELD']"); 

by id (if you know the field's internal guid, the dashes will ahve to be replaced by underscores:

// example field id, notice the guid and the underscores in the guid ctl00_m_g_054db6a0_0028_412d_bdc1_f2522ac3922e_ctl00_ctl04_ctl15_ctl00_ctl00_ctl04_ctl00_ctl00_TextField

$("input[id*='GUID']"); //this will get all input elements of which the id contains the specified GUID, i.e. 1 element

We wrap this in the ready() function of jQuery, so all calls will only be made when the document has fully loaded:

$(document).ready(function(){
 // enter code here, will be executed immediately after page has been loaded
}); 

By combining these 2 we could now set your dropdown's onchange event to the following

$(document).ready(function(){
 $("input[title='DISPLAYNAMEOFFIELD']").change(function() 
 {
      //do something to other field here 
 });
}); 

Would editing the fields in the alert template fix your issue?

http://blogs.msdn.com/sharepointdeveloperdocs/archive/2007/12/07/customizing-alert-notifications-and-alert-templates-in-windows-sharepoint-services-3-0.aspx

"To exclude some fields from being rendered in the email, you should include them in the <DigestNotificationExcludedFields> and <ImmediateNotificationExcludedFields> section."