Enable ASM policy based on pool selection in iRule

If I understood correctly what you are trying to achieve I think you need an iRule that will disable ASM for each HTTP request in HTTP_REQUEST and then selectively dis/enable ASM if LB has already taken place for a previous request on this connection. In LB_SELECTED selectively dis/enable ASM for the first request on any connection. The following iRule probably won't work as is but intended to be a pointer:

  when HTTP_REQUEST {
  if { [LB::server addr] ne "" } {
     Skip the leading partition name from the pool name:
    set pool [getfield [LB::server pool] "/" 3]
    set asm_policy [class lookup ${pool} map_pool_to_asm_policy]
    if { $asm_policy ne "" } {
    ASM::enable "/Common/$asm_policy"
    }

  } else {
    log local0. "New HTTP_REQUEST with no LB, disabling ASM so LB_SELECTED will always fire"
    ASM::disable
  }
}

when LB_SELECTED {
  if { [LB::server addr] eq [getfield [LB::server pool] "/" 3] } {
    log local0. "LB selected set [LB::server addr] - disabling ASM"
    ASM::disable
  } else {
    log local0. "LB selected set [LB::server addr] - enabling ASM"
    ASM::enable "/Common/$asm_policy"
  }
}