(By the way, "Persistent Includes" is a BBEdit thing. They're documented in the BBEdit manual in Appendix C: "Placeholders and Include Files.")
BBEdit stems from a period where you had to do everything from scratch (let's say, like under windoze). TextMate, on the other end, was developed with the clear intent of leveraging the UNIX text-processing infrastructure. Persistent includes have no reason to be in TM: we're all set with the bundling of a script like allan's (erb instead of php? php is... ugly!).
My .02€.
Domenico