No, there's no real way to zoom in... but what I do is I select the faces for a given group, and drag a corner of the selection so I can pick out individual facets, then select the single group, assign it, and hit the "" key, which toggles visibility. I then go through the assigning groups, turn all the groups back on with the "", then give it a single UVMap, and choose "Color by Group" to make sure I don't have any broken groups. -Then- I save an intermediary step. Finally, I give it the actual UVMap that is going to be used for the final model, and save it again.

Remember, kids! Napalm is Nature's Toothpaste!