Ok, finally managed to get some photo's sorted of the disassembly of the Joytech Williams F1 wheel unit and the attachment of the wheel to a Logitech G27 (need to check but I think it should be the same for the 25). The majority of the photo's are of the spring type Joytech unit but from image 18 onwards it shows the internal assembly and barrel of the force feedback type.
http://img585.imageshack.us/slideshow/webplayer.php?id=dscf0959x.jpg
Once stripped to the barrel the idea of using the two Stainless Steel discs to mount the barrel to the Logitech housing would be similar but I think that the barrel from the force feedback type would be slightly better and stronger. This barrel has grooves on opposing sides and the top one needs to be made deeper and a little wider, then an additional two grooves need to be cut so that the barrel slides down between the three screw mounting lugs of the Logitech housing (image 23). This is a really firm, secure mounting and there is no slip as the three mounting lugs sit tightly in the grooves.
This is as far as I've got at present as the G27 needs a new pcb. I have noticed that the hole in the bottom of the Logitech shaft, through the centre of the small worm gear, is not very large. The plastic sheath from the Joytech quick-release connector cable may need to be removed and just the wires taken through.
Connecting the Logitech pcb to the Joytech wheel/buttons will need to be done but this shouldn't be too difficult a task. Initially I will probably use the two lower shifter paddles as two of the original six buttons on the G27. If you have a G25 then you only have two buttons anyway.
Further work needs to be done now on how to use the multitude of buttons on the Joytech wheel. The original Joytech pcb uses capacitive effect, similar to a PC keyboard. To try and marry the original Joytech pcb would I think be a nightmare so currently I am looking at using a piece of standard Veroboard to fix small TACT switches underneath the buttons, so that the button sits nicely on top of the switch with hopefully little play, and wire these to the Logitech. An alternative to the TACT switches would be to fix the original buttons to small circular switches and glue these into place into each button shaft in the top cover. As with the use of Leo's SLI-M, SLI-PRO or the Symprojects displays this is still being looked at.
Hopefully this will give you some idea of the possibilities and provide a little help to anyone else wants an F1 type wheel without taking out a mortgage to pay for it.