Cast Iron baseboard on a Steam System
If the installation will require piping the baseboard to a dry return you could use a steam trap on the end of the radiation. The trap will allow air to pass through but when steam gets into the trap the trap will close. When enough condensate accumulates in the trap it will cool approximately 10f and the trap will re-open to allow the condensate to exit the trap and close again when the live steam enters the trap. An air vent on the radiation would not be required. Another option is explained below. A few installers have had issues with water from the vent with this installation but most have worked well. The few with issues I believe were operating above 2 psi which is the max pressure on residential systems.
If you do not want to use a mechanical trap on the return side of the baseboard and you have the room you could create a water seal between the baseboard and the dry return with a trap. Steam will not get into the return as it cannot get trough the water trap. Pipe the return side below the dry return and back up to create the required water seal. The water seal must drop 30" per pound of steam pressure below the return connection point.
This is the same as above but the wet seal is returning back to the steam main. I prefer this drawing less than the above drawing as much more water ends up in the steam main possibly causing water hammer noises in the system.
In this last drawing you would be piping the return side of the cast iron baseboard into a wet return verifying the connection will always be below the water level of the steam system. Since water will seek it's own level, the water in the system piping will be equal to the water level in the boiler.