We would not use the overlay to take wall thickness credit for additional pressure containment, but the overlay will stiffen the pipe. We model overlayed pipe as:

Caesar WT: base pipe wt + overlay thickness
Caesar CA: line class CA + overlay thickness

This will give us the right loads due to the extra stiffness and weight of the pipe and also allow us to check that our SUS stresses are OK after the overlay and small CA are gone. The only issue is that the mill tolerance will be a little bit larger than it should be so you may run into the warning message about not having enough wt for pressure (in rare cases) but we have been ignoring that. (I guess you can tweek the mill tollerance so that everything works out but I don't think that is worth the effort.)

Regards,
Dave Clark / J. Zbitniff