This reminds me of a technique some people are using: Delay responses by x seconds, where x is a constant. x=46 and x=90 have been mentioned as good numbers; apparently quite a few spambots drop off at 45 seconds or less, while hardly any proper smtp clients do.
I see no reason why your variation should have less success, and would be very interested in seeing a comparison if you're able to carry one out.
Arnt
