Can I use an orifice plate in finned steam convectors?

You could use orifices... but it's much easier and more reliable in most systems just to repair the traps (there's no need to replace them -- it's rare to find a trap for which repair parts are not available). Orifices, to function reliably, must be used with a rather restricted and very low pressure range for the steam feed -- typically on the order of a few ounces per square inch maximum or, alternatively used where the steam feed pressure can be controlled within a very narrow pressure band.

The orifices would work well if your boiler can operate at a fairly steady pressure less than 2 PSI. And if you are somewhat over radiated because the orifice method usually would only give you 80% of the max heat available from the emitter.

If you want to go for the trap repair, IIWM, I would remove the convector via the unions at each end.
Then with an impact driver and the correct 6 point socket the tops come off easily. Use a pipe wrench on the body for back up to prevent any riser pipe damage.
That would be a good time to take the fin tube outside for cleaning also.

We use Orifices nearly exclusively when faced with systems with radiator traps. Typically you can size them for very low pressure drops ( like 8 ounces) and often eliminate the condensate pump. This is especially the case for former naturally induced vapor vacuum systems that originally had no condensate pump. If sized for a higher pressure drop, like 2 to 3 PSI, you can use a modulating boiler with outdoor reset so in warm weather very low pressures are produced causing only a small portion of the element to heat and in colder weather gradually turning up the heat source to warm more of the heating element. With the higher pressure drops, you can get some slight noise in colder weather. Most of the time, however, orifices even designed for 2 to 3 psi drop on design days will only be running 8 to 10 ounces on a typical winter day. The other nice thing I have found is that for most vacuum systems, you can eliminate the vacuum pump too, since the orifices help ensure even delivery of steam to the radiators and, you can effectively downsize the radiators ( since they are almost always quite oversized) to the smaller piping found in vacuum pump system. The only place where they are limited is in convectors with air dampers or on systems that need the full capacity of the. heating elements. However, even on these systems using an outdoor reset control you will benefit most of the year because the steam will rarely reach the steam trap, giving you much longer trap life.