I just realized what your reasoning was, dc: temptation + capacious. Got it.
Mollusque, as you know, there are a lot of crazy spellings out in Googleland, especially on web pages written by people who do not have a firm grasp of English (I mean that English is a second or third language for them). I respect your checking the word on Google Books. I will have to remember that.
Rolig, I called it a variant rather than a misspelling because it gets more than 10% of the g-hits relative to temptatious. But on Google Books, there's only one bona fide usage (after excluding OCR errors), which means editors don't let it stand, so I agree that it's a misspelling. I added it to my panvocalic list after dontcry commented on it, but have dropped it now.
OK, dontcry, if you say so. But at the risk of sounding prescriptivistic, this seems to me more like a misspelling (or a "creative spelling" as one of my teachers used to say) than a variant of "temptatious."