(Edit: Sorry, but I have just realized, that my whole following post missed the point of your initial question concerning the control of the Env Attack Time with VELOCITY , because what you are asking for is not possible, but the following explanations might be helpful anyway, because you can control the Attack Time and even the Release Time of Filter and Amp, dependent on you playing style.)
You are fortunately wrong concerning a whole SN-Synth tone.
If you go to the LiveSet edit level and then to the Partial, resp. Tone edit level of one of the four available single SN-Synth (or Acoustic) tones, you will find under the Synth modify tab a field called „Time Interval Env Sens“ with the parameters „Attack Time“ and „Release Time“. (Here you will also find the very useful "Envelope Loop" function.)
So on the SN-Synth tone level (which means for all three Synth Partials at the same time) you get exactly what you are asking for, not only for the attack time, but also for the release time of Amp and Filter!
Quote from the Parameter Guide page 36:
"Time Interval Env Sens
Attack Time 0–127
Shortens the FILTER and AMP Attack Time (p. 39) according to the spacing between note-on events.
Higher values produce a greater effect. With a setting of 0, there will be no effect.
This is effective when you want to play rapid notes using a sound that has a slow attack (Attack
Time).
Release Time 0–127
Shortens the FILTER and AMP Release Time (p. 39) if the interval between one note-on and the next
note-off is brief. Higher values produce a greater effect. With a setting of 0, there will be no effect.
This is effective when you want to play staccato notes using a sound that has a slow release."
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