This is not actually "literally zero attack time" as claimed - it's simply "look ahead". The attack time is exactly the same as before! This is important because zero attack time can distort audio.
For example, Selig Leveler has true zero attack time, one of the only compressors I'm aware of that can claim this. However, when using pitched material distortion will be heard. This is why Look Ahead was added (along with some behind-the-scenes trickery).
The advantage to using lookahead is that you can leave the attack time slower than "zero" and still catch transients.
This "trick" works quite well in Reason with very little prep - in fact, even better than the example above in Pro Tools because you don't have to copy a track. This means you can send multiple sources into such a compressor without multiple copies of the audio tracks in the sequencer. Just one more way that Reason, despite being behind in some areas, excels in others!
Here I demonstrate how you can add look ahead AND side chain filtering to the Master Compressor in Reason using only two devices (three if you use the DDL-1 instead of a true stereo delay like The Echo, because you need two: one for each channel).
In this example I use DDL-1s because they allow 1 ms increments. Here's what the routing looks like:
I setup the insert programmer to control the crossover frequency with one Rotary, turn Look Ahead on/off with a button (by using bypass on the delays), and a second Rotary to control lookahead delay from 1-10 ms.
For this to work you must set the Stereo Imager to NORMAL (Solo mode), and select the Hi Band on the back for the second pair of outputs. You also need to make sure the insert routing for the Master Insert is set to "INSERTS PRE COMPRESSOR".
Basically the main output of the Stereo Imager passes the signal un-processed on to the delays, which add the delay (naturally). The non-delayed signal gets hi-pass filtered and passed to the KEY input of the Master Compressor. By delaying the main signal, the KEY input effectively "sees" the audio BEFORE it hits the Master Compressor, effectively creating the "pre-delay" required for Look Ahead to function.
Hopefully this all makes sense, and demonstrates how Reason can handle some fairly complex functions much simpler than other DAWs!