I have only had my 3800 a week but have successfully printed canvases from 6x4 inches to 17x24 inches. I cut my canvas of a roll of Breathing Colour Brilliance Chromata White. I find 6x4 sheets I cut usually just go straight through from the sheet feeder, though it's not a size I would usually print (I just had to try it!). Larger sheets need help. In a similar manner to what has been quoted above, I tape on a piece of paper as a leader. I just use a strip of double sided sellotape.
I have experimented with this and come to these conclusions:
1) it does not matter what paper I use as a leader - plain paper works, as does Ilford Smooth Pearl and Canon Photo Paper Plus Glossy - any "normal" paper seems to pick up.
2) A half inch leader is enough to work. It works if I keep the whole sheet or just cut a one inch strip and leave half an inch hanging out. Printing from either Photoshop or Lightroom, it is easy to add a half inch margin to the left or top to compensate for this.
3) The leader does not have to be the same width as the page but must extend to the right edge of the canvas, viewed from the front of the printer. However, with smaller sheets, the paper may twist a little if I don't match the full width.
4) This works best using one sheet at a time through the normal sheet feeder!
5) I am waiting for a custom profile but have found the colours very true using the Epson Enhanced Matte Paper profile. I did not expect to be able to print confidently from Lightroom but colour prints on canvas have worked very, very well.
6) I have not had to reset the platen gap to print canvas, which I had expected would be an issue.
Having said all that, I think it is absolutely ridiculous that Epson did not make this printer able to print canvas straight out of the box. The Canon IPF5000 I really wanted to buy (except it was twice the size and, in New Zealand, nearly twice the price) not only prints canvas but switches from glossy to matte without wasting any ink. Come on Epson!
A week in, I am stoked with the superb prints and surprised to be apparently using less ink than expected. Even the canvases are frugal and brilliant.