I find that regen can help significantly in stopping but it can't be counted on because of the algorithms used by Zero (e.g. when bike is fully charged and throttle is neutral, regen pulses but once charge goes down as long as speed is below 70mph regen is constant with neutral throttle).
Zero doesn't [or can't safely] enable regen with a full battery. It's just that the battery can't act as a load for the spinning motor when fully charged. Usually this only lasts for half a block, so just plan around that bit.
The thing you can control is regen below 15 mph, which is disabled by default for efficiency reasons (crude explanation: it's difficult to induce a load at low speeds without applying some current to the field). The firmware interface accessed by DigiNow's OBD-II dongle allows you to enable regen all the way down to a stop. The controller settings also include the upper speed where regen cuts off (70mph you've noticed).
I have been thinking about getting one of DigiNow's dongles but I will probably wait until after I get my Super Charger. Its good to know some of those settings can be changed using the dongle.
I was just giving an example though. It may not necessarily be Zero programming (it could be the SEVCON itself) but I have found that regen does not work consistently on my 2014 Zero S (maybe things have improved on 2015 and/or 2016 models?). Half a block is a little short for "pulse mode" to stop when at full charge. I typically see at least a mile before "pulse mode" stops with a neutral throttle...on streets with 30-40mph speed limits. That's mostly an annoyance. The REAL issue I have is in situations of hard braking. Most of the time regen kicks in just like I expect and it can REALLY boost stopping power. But sometimes it just doesn't work! I have had situations where the bike can be at 50% charge and I hit the front brakes and there is no regen. Thats why I say you can't count on it. I brake like its not going to be there and if it is then great I stop faster. It could be something specific to custom mode, its been a long time since I have ridden in Sport and I have only used Eco a few times just to see what it was like. I have also had rides where regen isn't there at all despite being in custom mode with torque and braking regen settings at 100%. Turn the bike OFF and then back ON and everything works normal. It wouldn't surprise me if these are two symptoms of the same problem...some kind of firmware bug in the SEVCON or somethings (maybe I need a firmware update?).
On a regen tangent....another thing worth noting that I haven't heard anyone mention is that regen can add just as much heat to the motor and controller as heavy acceleration. On a Zero, if you are just accelerating at max on every stop you may never even come close to a temp warning. But if you do that AND you are braking hard to come to every stop you can get to a temp light a lot quicker. Whenever I get a temp light I ease up so I have never gotten to the point where performance is cut back but I would assume that regen will be cut back too, just to be safe.