My favourite pen ran dry unexpectedly, and since I can't get another quickly I decided to try refilling it with inkjet ink. I also ordered some new pens from Japan via ebay because the same style of pen is sold there as a "brush pen" for writing Kanji characters. Theirs is much more stylish (cream body with Kanji characters on it) so I decided to give them a try.
Initially I thought the whole back of the pen was an ink reservoir (it uses a tank of liquid ink) and was originally hoping to just drill a hole in the end, fill it with ink and then use a standard inkjet refill plug to seal it. But it transpired that the rear of the pen is actually an empty void. It's notable that the back is coloured solid black to prevent you from seeing that it's not full of ink!
However, it's still quite easy to fill, albeit a bit messy.
Remove the end cap.
Use pliers to remove the tip body without crushing the writing tip.
Use pliers to remove the air inlet assembly.
Fill with ink and reassemble. The ink literally wicks back down for use in seconds.
And the result is a fully working pen with good black lines and a full ink reservoir.