Well, I was reading on another thread about how a Panogear user could not get the center of the lens over the pivot point because the rig was too narrow (apparently limited to 60mm dia lenses) so I think your 800mm lens might not fit that model. You would have to try it out to see before you bought one.
If you are planning to stick an 800mm up high then I would suggest that you are going to spend serious money on buying (or hiring) a very sturdy mast to put it on. The airspeed just a few metres off the ground can be quite a bit faster than at ground level, and can be very gusty in the lee of buildings, trees, etc.
If you want a ready-built model then read no further!
There are a number of home-made motorised pano heads built using steel or aluminium frames (such as http://www.youtube.com/watch?v=nWpOI2bGaBU)
which should be able to hold an 800mm lens.
Ideally, both the main frame and tilting frame should be closed shapes but, if you want the camera to point at high angles, a U-shaped lower frame would be necessary. If you use large-section aluminium tubing you can use thin walls to keep the weight down. Even a U-shape is much more rigid than a conventional single-arm head and would be a better option, even if you are going to be shooting in portrait mode (imffho).
It would probably be adviseable to use metal gears for both strength and rigidity. Worm drives are best because they are far simpler and do not 'run back' so you do not need to use motor power to hold the camera steady. They do have to be close-fitting to minimise wind-induced vibrations.
If you have a bit of programming experience then a Picaxe or Arduino controller would be best. They can control very powerful electric motors, preferably steppers for accurate control. If you want to use conventional motors, rotation counters (from an old printer perhaps) can be added. Tetrix rotation counters are pricy but read to 1/4 of a degree (this accuracy is multiplied by the gearing).
I have no facilities for metalworking so used a LEGO controller and motors on wooden and MDF frames, but for a large lens would prefer a metal frame and more substantial motors.
The LEGO NXT has three motor ports to drive the motors which come with the kit. Using devices such as the Mindsensors motor/ relay drivers allow you to control very powerful conventional motors. A port-splitter would let you control one such motor from each port while using the rotation counters in the LEGO motors to keep track of the rig.
Alternatively, the sensor ports can be used to control stepper motors through cheap I2C chips (or Arduinos if you prefer).
Very complex mechanisms have been created with the NXT-G language which comes with the kit but there are quite a few alternative languages available, many of them free. I prefer NXC, which is not only free but continually upgraded. Have a look at the Mindboards site if you are interested.
Although most programs run on the NXT, some are designed to use the inbuilt Bluetooth connection for remote control from a pc, laptop, pda or even a mobile phone. You can have it both ways, keep a program stored on the NXT for normal use and another on your laptop for when remote operation is required.
Because you are writing the program, the rig can be set up to work just how you like it. You can have one program with many options or a set of smaller, specialised programs.
My own program has options for bracketing, mirror lock, logfile, etc. and is continually being modified; I am about to add a GPS module to record the date, time and location of each pano to the log file. If I fitted the GPS to the rig I could record the direction of each shot if I wanted. A Mindsensors motor/ relay driver controls both focus and camera shutter on my 400D, but a couple of cheap 9v relays and diodes will do the job just as well. Or I could control both from an I2C chip.
Building your own kit takes a bit longer, but you have complete control and can change things if they do not work the way you want. You can also repair it yourself, unlike a factory-built unit.
And it is MUCH cheaper. EOR (end of rant).
Update, forgot to say the if you are looking for a 2-camera setup then it would probably look something like this -