iRule for selecting ASM policy

another option is an ltm policy. I'm guessing here on how you determine (going with the Accept-Charset header, but you could also match on other headers on request or response):

ltm policy cpm.charsets {
    controls { asm }
    requires { http }
    rules {
        default {
            actions {
                0 {
                    asm
                    enable
                    policy /Common/asm_default_policy
                }
            }
            ordinal 3
        }
        win1252 {
            actions {
                0 {
                    asm
                    enable
                    policy /Common/asm_win1252_policy
                }
            }
            conditions {
                0 {
                    http-header
                    name Accept-Charset
                    values { Windows-1252 }
                }
            }
            ordinal 1
        }
        win1256 {
            actions {
                0 {
                    asm
                    enable
                    policy /Common/asm_win1256_policy
                }
            }
            conditions {
                0 {
                    http-header
                    name Accept-Charset
                    values { Windows-1256 }
                }
            }
            ordinal 2
        }
    }
    strategy first-match
}