Curiosity got the better of me and I had a look to see if this was actually archived – it turns out there are several snapshots of the site up until 2010, this is the earliest – from 2005.
https://web.archive.org/web/20051220140629/http://www.livewireunicycles.com/Outta%20Phaze.htm
An old but very interesting blog post by Bruce Dawson has a photo of Purple Phase, and examples with details of various methods of gearing up a unicycle. It was probably written around the same era that this thread started.
That’s where I got the idea for my uni, I want to try to make a kit that you can attach to your unicycle to acquire gears
Thanks for the link! Here’s a whole section that’s really fun nowadays:
Why don’t derailleurs work?
Riding a unicycle is, like walking, a constant state of near falling.The rider needs to perform tiny adjustments many times per second in order to keep the center of support directly underneath the rider’s center of mass. If you start falling forwards then you need to instantly pedal faster. If you start falling backwards then you need to instantly push back on the pedals. These corrections are happening multiple times per second, usually without the rider even noticing. It’s only when the corrections almost don’t work that the unicycle perceptibly moves from the vertical and the rider’s heart rate goes up a bit.
The derailleurs which are used for gearing on bikes don’t let you apply back pressure. If you pedal backwards you freewheel, and if you stopped the axle from freewheeling then pedaling backwards would just damage the gearing system.
It is possible to ride a unicycle without being able to pedal backwards – doing a wheelie on a bike that freewheels proves this – but it is not stable, and it doesn’t work downhill. Technically it is possible to use a brake as a substitute for pedaling backwards, and anybody who thinks that that is a good idea is welcome to try it.
Since the time the article has been written, it has been proven that freewheel can be more than just toys. Think of toutestbon and his coast of more than one mile. Also, think of people like Becky or Florian ‘Freewheel’ Kaiser that can ride their freewheel down a highly technical trail.
Good point - I first read the article some years ago and hadn’t come across freewheel unicycling then, so was quite happy to go along with Bruce’s conclusion. Having since realised such things not only exist, but are now being ridden in all sorts of situations (admittedly some of those only by the unicycle gods!), it no longer holds true. It’s one of those times when the ‘impossible’ things of one generation are conquered by (some of) the next.
Re geared unicycles, does a proficiency at freewheeling open up the possibility of a derailleur gear change mechanism on a unicycle after all? Not simple or pretty, but might enable more than two different ratios to be available. Way above my skill set!
If anyone ever builds such a uni, I’d be very happy to give it a try 
You need a reasonable length of chain for a derailleur system to cope with the misalignment of the sprockets and chainring; also you would need a single chain system otherwise both sides would have to shift in perfect synchronicity which might be a bit of a challenge.
So basically you are looking at having the cranks above the wheel so you can get enough chain length and also have a single drivetrain – so a multispeed freewheel giraffe…
However, that is maybe a bit restricted of a viewpoint – it is not a million miles away from taking the front wheel a fork off a bike (leave the handlebars) and wheelee-ing/manual-ing that…
Freemounting it would be a challenge for sure 
Run, jump, and voilà! You’re on the beast
I would be afraid to break the derailleur, though. It would probably not be really solid.