My favourite mechanical pencil, the Fixpencil 22 is a joy to work with, but increasingly I want to work digitally on my iPad. My second favourite "pencil" is the Adonit Jot Touch, which is a really neat pressure sensitive bluetooth gadget. Still, my favourite iPad drawing app is Paper by FiftyThree, which doesn't support the Jot Touch bluetooth features.
The Jot Touch is kind of heavy - it has much more momentum and mass than any other pen or pencil I regularly use, so I became curious if one of the jot touch tips would work with my fixpencil - and hey, it does! It works amazingly well - the whole pencil is made of metal except for the plastic button on the end, and you can even leave the pencil lead loaded in behind the jot touch tip.
Just one thing - the Jot Tip's are kind of fragile - not the kind of thing you'd want to leave exposed to the world in a backpack for weeks and weeks, so I designed and printed this little endcap to protect the jot tip while it's loaded in to my Fixpencil.
The OpenSCAD file also includes a simple model of the Fixpencil itself, so you can use it to create your own accessories, including an untested parametric model of the end button, as measured by a micrometer, so you could print out spare buttons or create buttons with different designs - but the model doesn't include the lead sharpener part - I bet you could make one in plastic though!
I printed mine on a Makerbot Replicator 2. I found the 'High' setting took for fricking ever to slice. So, choose the 'medium' profile then set the layer height down to 0.15 and it'll slice in a few seconds and print nicely enough. ^_^
To make the stylus itself, pick up a Caren d'Ache Fixpencil 22 - these are pretty popular, so your local art supply store probably has some in stock. Also order from Adonit's Spare Parts store a set of replacement Ball Tips - I used the spare included with my Jot Touch, but the Jot Flip and Jot Pro tips would probably work great too. You'll also need to order some "spare" discs. Once you have all these parts, get the ball tip and insert it in to the Fixpencil like a graphite stick with the ball facing out, then press the ball in to the little cup on a plastic disc. Hey presto! Nicest iPad stylus ever! All together you're looking at spending about $50, but you will have an excellent iPad stylus and a great art pencil when you're done.