I haven't used the pentel brush pens, but I have tried reloading several types of disposable tech pens by hand. I've successfully reloaded sakura pigma tech pens and the faber-castell Pitt markers.
I usually wait until I have a handful to do and then do them all at the same time. I secure the pen in a small vise and then use pliers to pull the end cap off (they come right off). I then use Higgins calligraphy ink and a cheap syringe that I purchase at the hardware store to inject the ink into the sponge-reservoir. It doesn't take much, so don't go crazy! Put the cap back on the end of the pen and give it about 2 hours to check for leaks. If they are leaking I throw them out (I was going to throw them away anyhow).
The Pitt brush pens work best, and best of all the tip in them is reversable. Use a paper towel and pull it straight out and flip it over and put it back in. Works sweet.
I know, I'm cheap, but at 3 bucks a pop I like to stretch the life of those pens out as much as I can.