Hi, I create a simple code to try to understand how netsuite manipulate form data. Particularly I'm interested on 'customform' field.

Test:
   Custom Customer Preferred Form has id = '2'.

   Create and edit a customer with a different customer form with id = '4'.

Before Submit Function Script Code:

var rec = nlapiGetNewRecord();
var custForm = rec.getFieldValue('customform');
nlapiLogExecution('debug','Before: Custom Form',custForm);

After Submit Function Script Code:

var rec = nlapiGetNewRecord();
var custForm = rec.getFieldValue('customform');
nlapiLogExecution('debug','After: Before Load Custom Form',custForm);

rec = nlapiLoadRecord(rec.getRecordType(),rec.getId());
custForm = rec.getFieldValue('customform');
nlapiLogExecution('debug','After: Custom Form',custForm);