I went ahead and marked @scpeters' answer as correct, because it does answer my original question. The thing I left out of the original question was that I need to spawn URDF models (and would prefer not to convert them to SDF). @AndreiHaidu's answer inspired the solution I actually used, which was to modify drcsim's gazebo_ros_api_plugin to provide the spawning services. It turns out that the lines to provide the services were present, but not being compiled due to dependency issues that would arise when compiling drcsim. (I discovered later that I should have used the simulator_gazebo version of gazebo_ros_api_plugin, because it has some bug fixes that the drcsim version doesn't.) Then I was able to use the modified version of the plugin with the system version of Gazebo to spawn URDF models via ROS.
I went ahead and marked @scpeters' answer as correct, because it does answer my original question. The thing I left out of the original question was that I need to spawn URDF models (and would prefer not to convert them to SDF). @AndreiHaidu's answer inspired the solution I actually used, which was to modify drcsim's gazebo_ros_api_plugin to provide the spawning services. It turns out that the lines to provide the services were present, but not being compiled due to dependency issues that would arise when compiling drcsim. (I discovered later that I should have used the simulator_gazebo version of gazebo_ros_api_plugin, because it has some bug fixes that the drcsim version doesn't.) Then I was able to use the modified version of the plugin with the system version of Gazebo to spawn URDF models via ROS.ROS, though Gazebo is displaying one of the models 100 times smaller than it should be... (But that's another issue.)