A custom profile might very well improve this situation. Unfortunately, this requires an expensive bit of hardware I don't own; or paying someone else every time I want to use a different paper. I did some research to determine the media type used by third party paper manufacturers who offer ICC profiles for their paper. Every single one of them used a media type that enforced the 1.38" margin and prevented the use of custom paper sizes. In other words, I did not find a single instance where I would not have to pay for a custom profile to avoid the restrictions imposed by Canon's driver.
I've just run into this problem having bought an expensive set of Hahnemuhle photo rag and museum etching papers - the fact that the maximum print size on A3+ paper is quite a bit less than a borderless A3 print is very disappointing.
Would a custom profile really solve this problem? I didn't think the profiles contained this sort of restriction, but my knowledge is very limited! Can anyone who has profiled some fine art paper confirm? Like t.linn, this is the printer I've been waiting for and am prepared to spend the extra £300 for a munki if it's a proper fix.
Ultimately Canon needs to remove these restrictions in their software to give us the choice.