Matt,
There are two things potentially in play here:
1. As John mentioned, borderless printing is not supported on all roll widths. Since the printer measures the paper, the only workaround here is to use different paper.
2. The media type you've selected. Along with the rest of the black magic in the iPF8000 media selection is a flag that decides whether or not it's "safe" for borderless printing. If Canon has determined a particular paper isn't suitable for borderless use, borderless printing is disabled. You can work around this by selecting a different media type.
Do be aware that in some cases Canon has opted out of borderless because of the potential for head strikes. I wouldn't try a huge print right out of the gate.