A few things:
1) First, "urgent care" is far too diverse to make any general statements. At one extreme, a hospital owned clinic, or one the larger, more reputable groups will typically only hire EM board certified physicians. At the other extreme are clinics that hire any physician with a valid license - and aren't real picky about that. The question you need to answer for yourself is if you would feel competent picking out the truly sick kid out of the 100 trivial cases. I have my own thoughts about that...
2) There are a number of large, industrial facilities that have in-house "urgent care" clinics. This would be the natural environment for an occupational med physician. In addition, many hospital systems have dedicated occupational med clinics. The one in our system basically does walk in DOT/employer physicals along with "urgent" injuries. There should be an opportunity for part-time work in both of those.
3) I know a few former flight surgeons who have a nice gig doing FAA physicals. The opportunity would obviously be dependent on location, but this is the type of thing you really need to build by "word of mouth" advertising. It is not the type of thing that you can move to a new area, but an add in the Yellow Pages, and expect patients to show up the next day. On the other hand, if you build up connections in the civil aviation community and are planning to stay in the same area, it can work out as a nice niche opportunity.