Probably depends on how you define "modest" - are we talking less than 1 watt QRP or 4 watt carrier CB power levels or QRPP/Part 15 power levels? If those emergency CB radios mentioned by Kage used RCA jacks then probably. I remember I used to have a late 70s vintage Radio Shack (Realistic) handheld CB with a massive center telescopic whip antenna and it had connectors on the side for external microphone, speaker, 12VDC power and antenna...and the external antenna connection was a RCA connector. The handheld CB was wither 2 watts or 4 watts transmit power rated, that's with a fresh set of batteries or an external 12 volt power supply though.
Did I ever actually connect an external antenna to the handheld CB? Nope. But as mentioned by Kage, there were other radios sold with these connectors so they must work to some degree or another. I imagine that the emergency CBs aren't designed for absolute maximum range but the antennas still need to present a workable impedance and SWR to the transmitter.