I am an anti-smacker with a strong-willed dd. Tantrums are a common feature in my house, some days worse than others. Agree with zippi that heading off tantrums is better than dealing with a full blown one when dd has effectively lost control.
In the scenario you described, you acted reasonably and dd still went "off on one". The smack seemed to work but I don't know whether that will create a vicious circle in the long run. Granted your dd was already worn out and needed a face-saving way to stop. My mother caned me for fighting with my siblings and although I hated her for it (threatened to report her to the authorities!), I remember in some perverse way, I would escalate the fights just to get that response from her. So that I could hate her more. How mixed up is that?? A smack is not caning (I do appreciate), but I remember it was the unfairness of the situation (ie mother strong, me small and weak) which made me very bitter.
Anyway, you asked what anto-smackers would do. I ignore dd and after, say 5-10 mins, go to her and try to comfort her with cuddles/alternatives. She is probably afraid of her emotions and needs your help to calm down. If it does not work, I put her down and ignore her for another 10 mins. Repeat for as long as necessary. She will burn herself out at some stage (yes, could take hours) but at least you got the message across that you did not give in but you love her anyway even though she was tantrumming.
If her behaviour was about testing limits, this would go down in her experience that mummy stays firm. If it was because she was overwhelmed by her emotions, this would go down as mummy will help me get over it and loves me unconditionally.
I do admit to yelling at my dd during these times as well - hey, not perfect. But at the end of it, always cuddles and reconciliation. JMO.