You can just hit Enter or Return on those dialogues if the button is glowing blue (or grey, for those with the grey theme turned on) to select that button.
I'm pretty sure Kensington's MouseWorks software supports this, though I haven't used it since before X came out, so maybe this feature has been removed. It's worth researching if you really want it.
At least I'm reminded this is a mouse issue -- not OS X. I checked my Kensington Mouseworks System Preference and there is no "Snap-to" option available.