Generally battery life is increased with reduced stress, and stress for a battery is current (among others). Reduce the current (both discharge and charge) and you increase your battery life.
Chris Stratton points out very well that the main limitation you face is that the charge circuit inside is controlling the charge current to the battery.
So the voltage is basically irrelevant. It needs to be stepped down in any case (4.35 V maximum cell voltage for all lithium based chemistries I know of, most are 4.2 V).
Now with the current capacity of your wall adapter you can have some influence. Usually the device has to negotiate how much power it needs if it needs more than 100 mA. Well most of the devices simply take 500 mA. Above that it should have some sort of protocol or charger detection.
So you can reduce the charging rate if you select a slow charge mechanism either in your phone or with a 500 mA max rated charger, so the phone has to use the slow charge mechanism. With this you'll end up with a reasonably small charge current (in the order of 1/2 C).
Most benefit to battery life would be a reduced end of charging voltage like 4 V instead of 4.2 V, but as this reduces the run time of products significantly, probably no manufacturer employs this for a phone. You'd have to unplug it at 80% charge or something to get this benefit.