looking to see if there's a way to after a registration (possibly after first page full of info) to process some code to enforce certain rules and combinations of options etc.
aka i want to enforce a type of discount coupon also being used with a certain pulldown type in custom fields..
can the add-on for php scripts do this or is that just for post processing of a registration and cant effect it?
There is no way to process rules as you are describing. You CAN create conditional custom fields... so based on the choice selected in Field A, the user then sees Fields B and C. You can apply different fees/discounts through those custom fields, so that may be the direction to go. However, there is no tie to discount codes being enabled/disabled or processed differently based on any custom field values.
The MyScript plugin processes code AFTER a registration is completed... can't be used to modify how the actual registration fields / codes are handled.