Hi Shaun,
My first recommendation would be to contact your SIP supplier, as I suspect such an installation would void the panel warranty.
I'm assuming by SIPs you mean panels with OSB faces both sides. I wouldn't even think of hanging curtain-wall panels [sheetrock inner face] that way; the sheetrock is in no way structural, and couldn't be counted on to bear any weight. Even in panels with two OSB faces, the unsupported weight, primarily of the outer skin, will not be adequately taken care of by the nails or screws, which will tend to bend, deflect, or shear. Any movement could cause delamination and failure within the panel.
I think the only way to support the panel edges in your case would be to frame the deck with an overhang. Your local Building Inspector would probably require an engineer's stamp on such a modification, and it's likely to lead to complex [read expensive] framing changes.
Good luck, Leon
