I figured out how to print a single #10 envelope without having to remove paper from the bottom cassette. In my example I used MS Word. Set up the paper size in Word to 5x7 printing in landscape mode. The 5x7 size will trigger the paper to draw from the upper cassette. You need to lay out the address and return address on the 5x7 page but there is enough printable space to make it work.
I know #10 envelopes don't fit into the upper cassette - or do they? Turns out if you set the paper guides to the 5x7 size and the width to match your envelope you can fit a single envelope into the cassette even though it overlaps the paper guide and onto the frame of the tray. Just be careful as you insert the tray to keep the envelope against the far end of the tray. Once the paper is into the printer there is enough space above the frame of the tray to accomodate the envelope
Clearly its a tight fit and its a hack but it works and overcomes a major shortcoming of this printer. This is a single envelope only solution but for many of us that is enough.
A shame that Canon has forced us to go to these lengths to simply print an envelope when they could have designed the upper tray to accomodate an envelope.