Yes you can do 8 Ply on a standard gauge knitting machine!

My manual said you could knit with thicker yarn and I believed it, I just had to try it.
I left every alternate needle out of action and loosened the tension on the mast.
I started at tension 7 on the dial and ended in tension 4. The carriage was still easy to move, so you probably could to a smaller stitch again, but I liked the tension 4 stitch.

This is wonderful news for new machine knitters IMO as you can buy the sport weight or 8 ply 100gms for like $1.99. No need to worry about throwing away money on more expensive yarn when learning... anyway not going to blabber on. here are the pictures "after":

This is the purl side

This is the knit side

And just because I can, a poor quality video showing the experience... sorry I only used the crappy camera rather than the video camera as husbie is home on holidays and I needs his computer for the fire wire card to download the video camera... anyway here tis :)

 
As you can see my voice is still all silly like from the thing I had.. yay! for things hanging on in there! he he