<meta http-equiv="pics-label" content='(pics-1.1
"http://www.icra.org/ratingsv02.html" l
gen true for "http://www.example.org"
r (nz 0 vz 0 lz 0 oz 0 cz 0))'>
Where
www.example.org is your domain.
This is a generic PICS record that says that content may be present -- IRCA's system requires explicit classification of the content presented. Safesurf also uses PICS but has different ratings which are somewhat more generic, although, neither has a policy that states, hey, just mark it as adult. Both are a bit too granular to be simple, but, somewhat well supported.
I think the answer is rather evident. All of the safesearch filtering that I can find all use PICS. All we need is 1 location to host a simple policy file that can be defined or utilize an existing bureau's definitions. So, perhaps the rating=mature which is somewhat widely supported and the PICS tag with a bureau that makes it easy to generically tag pages.