There is real danger to occupants and parts of the building from building up pressure to unclog a pipe. It may also not be useful to try it. Unless there is a check valve in the condensate return, the pressure would build up equally in both supply and return and force return condensate up the return pipes.
Check valves are used in systems where there is no Hartford Loop on the boiler and they become dirty and fail to work unless they are maintained frequently, so one is most likely not in the system. Building up pressure in a Hartford Loop boiler would force the condensate up the return. High pressure can just as well let a weak section of pipe burst instead of a clog, damaging the structure.
A most likely reason for the cold pipe is that a vent at the top of the riser is not functioning or has been plugged. It is possible that the only vent in your system is a main vent or float and thermostatic trap near the main; then a clog or a water seal is at the bottom of the riser.
A productive way to find out is to find the bottom of that riser and examine it for dirt that has fallen to the bottom of the run. An intelligent way of piping an apartment riser was to put a cleanout and drain on the bottom. Look for that first. Open it and clean the bend. You already have an idea where it is coming from as you can feel the pipe as it rises through the floor levels.