The updated CO provisional rules do allow for SAO, DA/SA and DA guns with magwells.
The provisional rules do need to be cleaned up regarding language.
But the way to figure out allowed mods: Rule 1.1.2.A refers to an unmodified gun in SSP, ESP, CDP or CCP.
You can then modify it according to the CO Provisional rules as listed.
Custom guns get kinda tricky. It will depend on the type of mod. If you have a question about a custom gun for CO, please write your Area Coordinator.
The good news is that SAO and DA/SA are now legal....that made a lot of people happy.
http://members.idpa.com/Content/Rules/a3nozfcz.rcs.pdf
Cody