The soft tyre rubber is perfectly workable/turnable, using the "Flap" disc system.
You need to use the lowest grit (30 or 40) to cut to rough shape, 2mm or so from the bushings, them using the 100 to 120 grits you will get a close final shape and a quite smooth finish, where you only need to do your normal hand sanding to get it really smooth.
There are 2 things that you need to keep in mind, void the rubber from getting hot, this will be achieved by not forcing the cutting grits into the rubber nor leaving them there for too long. The hand sanding done with wet/dry papers will address that issue well...!
You may also want to consider what type of finish you want for the rubber surface, if very smooth with a gloss type finish, you need to use flexible CA, otherwise the finish will shatter, latter on, pretty much the same as it does when you coat leather with the normal CA...!
You may also like the finish the Flap disc with the coarser grits leaves on the rubber, in that case, you only have to coated with something. Try it out in a piece of tyre rubber first to see what I mean...!
One of the major difference of following Ed's suggestion, apart from being hectically wrong (in my view) sorry Ed, you wouldn't have the effect the layers of rubber glued together will give, as a piece of rubber hose will show no layers...! The rubber hose idea is excellent tough, if someone wants to try making a normal rubber pen.:wink:
If those people, want pens made out of those specific tyres, that is what you should use and do, if that isn't your coup of tea, for whatever reason, let them know that, with all honesty. Suggest someone else you believe will be capable to do the job or offer the job to anyone interested here, I reckon a few nearby will be happy with the challenge...!
That's what I would do, anyways...!:wink::biggrin:
Good luck.
PS: If you are not familiar with the "Flap" disc system, there is already enough info about this in this forum, some just
here
Cheers
George