custom config processing

[Sam Steingold]

Is there a way to add custom config file processing to config.status?
specifically, I want to insert a custom line _before_ each line matching my 
regexp.
i.e.,

$ sh config.status foo

should generate foo as usual and then do
sed -i 's/ZZZ/AAA\n/ p' foo
(or whatever -- what?! -- should be done to transform
===
a
ZZZ
b
===
to
===
a
AAA
ZZZ
b
===)

thanks.