Printing using different canvas size in LR 3
Find a custom lab, preferably local, that will do the print for you at the size you want. You will still probably have to pay for the next paper size up. I do this for clients all the time.
You could buy cheaper software that will allow increasing the canvas size. Photoshop Elements is a lot less expensive than Photoshop and will probably do it. There are probably lots of free software apps that will do it, maybe Picasa?
I don’t know why printing to jpeg should cause any problems with color shifts. You might want to look carefully at whether to apply any output sharpening or not. If you are worried about color shifts, you could send the a smaller version to print before the expense of the large print.
In what color space it is the lab expecting you to send the image?
After chatting with a local photography buddy, I realized I'm being somewhat dense in my request.
I can simply process/export my file as I normally would for a 2:3 print, let my usual lab print it at 2:3, and then crop with a sharp knife (very carefully) when the print gets back to me. It's just as easy to cut off a strip of blank photo paper top & bottom as it is to cut off a strip of unnecessary imagery top & bottom. Your post is correct - after polling a few labs, I'm typically going to pay for the 2:3 image anyway, so there is no cost difference.
During my conversation, it was pointed out that poorly managed sharpening can often cause color shifts (and a whole host of other output issues) as you describe. Perhaps the articles I read were folks who didn't realize this, but I thought I polled a wide enough variety of sources to overcome "limited knowledge" postings.
Thanks for your response; I'm learning all the time.