iRule for missing or blank headers (user-agent and accept)

It's slightly less specific but this should work (note the HTTP::header command will return null if the header doesn't exist or if the header's value is null);

if { [HTTP::uri] ends_with "rss" or [HTTP::uri] ends_with "smil" } {
 log local0. "URI extension is rss or smil." 
 Optionally drop the traffic
 reject 
 Stop executing the irule
 return }

elseif { [HTTP::header "User-Agent"] eq "" } {
 log local0. "User-Agent header is missing or blank." }

elseif { [HTTP::header "Accept"] eq "" } {
 log local0. "Accept header is missing or blank." }