new runtime variables

I am experimenting with some new datatypes that require some
metadata to control their action (e.g., like the DATESTYLE does for
date/time types). It seems that some new runtime variables make most
sense for this, though other suggestions are welcome.

Is there any documentation on how to implement runtime variables like
DATESTYLE? Presumably, I need to implement actions for the SET/SHOW
commands and some mechanism for retrieving values within backend code.
Anything else? How is this accomplished?

Thanks for your help.

Cheers,
Brook