There is a way to do it. I don't know why the native software won't let you (I have the same printer BTW), old thread but worth contributing.
Take a look at Print Fab, it's a RIP software (allows you a lot of control over how your printer behaves) and yes it's compatible with this printer, a lot of people use it for separations, profiling, control printing, screen printing, etc. BUT in this case it could be useful for you regarding custom sizes.
The software allows you to create a "mirror" secondary (and many more) printers and set custom paper sizes, and there you can create your own. It's useful because you can create sizes not supported by your native printer driver. I haven't checked or tried printing envelopes on mine because I don't need it and I don't have any envelopes to try it, BUT I can tell you a lot of people use Print Fab to create banners, that's long panoramic prints on printers that don't support larger paper sizes than the common legal size or 11x17 in the given case.
It might help you. The rest of what Print Fab can do for you has nothing to do with your situation other than the print size, give it a try, there is a try out available. If it works or not, please come back and share as it will help others.