How large is your text file (how many lines)? Is the desired result to have a sequence of events that you can loop and play at different rates?
One thing to keep in mind is that Smalltalk in the Script (or in a parameter field) is evaluated before the Sound starts playing. That's why it isn't letting you using an EventValue to control the rate within the Script (since that Script is evaluated and gone by the time the Sound starts playing).
It may be possible to use a Script to create a StepSequencer with your data-driven events in it; then you could play the StepSequencer at any rate and modify the rate on the fly.
Another alternative would be to evaluate the Smalltalk live in a Tool and play as you go.
Otherwise, you could just make multiple copies of your Script, each with a different MM value.
Let us know how many events we're talking about; there may be an alternative solution.