using iRule for health monitor type information for decisions

The easiest way to do this would be to simply keep your monitor in its working configuration and use an iRule to filter out the users. This would make it so if the iRule is applied, only tester IP ranges can navigate to the page and everyone else gets redirected. The iRule would look something like this.

when HTTP_REQUEST
{
    if {![class match [IP::client_addr] eq "TesterList"]}
    {
        HTTP::redirect https://example.com/redirect_page
    }
}

If you want to trigger a change from a remote server that would limit users, you are most likely going to have to write an iControl script. Here is a link to information on iControl so you can research further if you please.