So i got this to work finally, had to upgrade GSS to 3.2 to get the HTTPS HEAD feature (replacing with GTM hopefully later this year anyway!). Here's the iRule i used to give both GSS answer and maintenance page redirection.
GSS needs to be set to HTTP/HTTPS head, and set the path as /gss. If anyone sees a more efficient way then feel free to chime in.
when HTTP_REQUEST {
if { [HTTP::uri] starts_with "/gss" } {
if { [active_members [LB::server pool]] >= 1 } {
HTTP::respond 200 Location "http://"
} else {
reject
}
} else {
if { [active_members [LB::server pool]] == 0 } {
pool maint_page
}
}
}