The finished product.
This album includes modifications originally suggested by dogwan and wje at head-fi.org. Props to them.
One of the reasons I wanted to recable my T50RP was to get rid of the 1/8" TRS jack in the left cup (it looks like a little junction box inside the cup). After I cut the jack out, I filled the hole with a 3/8" hole plug (circled in red) from Home Depot (image: http://www.homedepot.ca/webapp/wcs/stores/servlet/ProductImagesView?D=955811&catalogId=10051&langId=-15). I had to ream out the hole to fit the plug and bent the claws on the plug back to secure the plug in place. I filled the hole left by the wire that runs to the right cup (blue circle) with a bamboo BBQ skewer whittled to size.
To seal the holes where the cables enter the cups, I used rubber grommets (circled in red). I drilled 1/4" holes and used 1/4" grommets (image: http://www.homedepot.ca/webapp/wcs/stores/servlet/ProductImagesView?D=956598&catalogId=10051&langId=-15). Note that I put the cable-entry point at the rear of the cup. If I were to do it over again, I'd put the holes near the front. The cable would rub against my shoulders less and I'd get less microphonics.
Here's what the cable looks like inserted through the grommet. When I shrunk the heat shrink in place, it created a ridge or bulge where the paracorded ended. It was a bit of luck because the bulge could be lubricated with water and squeezed through the grommet. When it dried, it anchored the cable in place and sealed tightly against the grommet. HOWEVER, if I were to do it again, I'd leave the twisted pair naked (without paracord but with heat shrink), to reduce the microphonics. In this case, I might need to use a small cable tie to secure the cable in the place.
I soldered the cables so they were oriented towards the bottom of the cups. I left about 4" of slack in the cable so it's easier to remove the baffles and make mods in the cups.
With all the cabling entering the cups from below, I no longer needed the cable that extends through the headband from the left cup to the right one. Snip. Snip. And, as shown earlier, I filled the hole with a piece of bamboo skewer.