Thanks - nice idea indeed, though you can never trust such improvisations when it comes to sensitive, expensive equipment... Talking for myself at least. You can never know whether there's some tiny piece of metal that could cut off of that 'plug', and fall right inside your device... Murphy's law
I also thought about covering it with a piece of tape, preferably something transparent, but I think there should be something out there that was planned for that.
Plugging dummy plugs would also be an idea, not the smartest one though.
The case I'm using for my recorder won't allow me to close it tight to the connected microphone plug, there's some space left which leaves another socket uncovered. Could be a risk when raining for example. Let alone if you choose to use the internal mics. The recorder is a PCM-M10 by the way.