Not sure if anyone caught this, not sure when they allowed it but you're now allowed to /targetfriend, /targetenemy, not sure any others one time in a macro. meaning..

/targetenemy
/cast Siphon Life;

That way you can spam that macro to cycle through nearest enemies and siphon life them. Of course probably want to reverse the order and do some check on your target to limit errors, that way you could cast it on who you're targeting and cycle to the next mob. something like this maybe

/cast [harm,nomod] Siphon Life;
/targetenemy

That wya if you can harm target, and you're not pressing any mod, cast siphon then target next enemy. That way if you already siphoned mob you can mod it and skip.