[rlisin]

I ordered small 2.5 and 3.5mm sockets (a pair + 1 spare of each). If one of them fits, i will try to install it; looks like the 2.5 mm should fit, the 3.5 - not sure. If neither fits, I will go with fixed cable. That can be done without any problems, and without losing seal - the connectors and the drivers have separate compartments.
 
I took some photos of these cans disassembled, I'll post them once I'm allowed to (not enough posts yet).

 

[rlisin]

I did it! I used the smallest 2.5 jack sockets I could find, 30 cents a piece. Nothing got damaged in the process (not even the old MMCX connectors), no drilling, gluing was necessary, I just removed the old connectors, soldered the new ones and that's it. The mod is fully reversible. Took me a little over one hour.
 
Fixed cable would be even easier to do, but I like the idea of detachable cable, just not one that makes the headphones useless
Plus, I already have a few cables terminated with 2.5 mono jacks for Sennheiser HD700s.
 
And now I can use the Onkyos without them disconnecting all the time, hell, I can headbang with them on! And if the cable goes bad, I can make a new good one for about $10, or buy a cheap cable (like the one in pics below) for even less. 
 
 
If you want to have a look at a step-by-step guide with photos, it's here:
 
http://audio-room.net/onkyo-es-fc300-step-by-step-modification-of-cable-sockets-mmcx-jack-2-5-mm/
 
 
And a few pics:
 

 
 
 
 
 

 
 
 
 
 

 
 
 
 
 
Here's the end result. Now I need to make a nice white or at least silver cable
 
 

 
 
 
 
And BOOM BIDDY BYE-BYE ridiculous Onkyo cable and connectors!