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The sugars in the syrup could scorch if chamber temps get too high for too long, so keep it low & slow, unless you're into blackened meat. Never tried it at the start, but I have glazed ribs with real maple syrup the last 60-90 minutes...good stuff when used appropriately.
Definitely agree with Eric. Need to be careful of sugars over caramelizing. I've heard people using honey as well. I think with a loin you'd be ok since you aren't cooking it for a really long time. I'd be curious to know how it turns out though!
It's a coin-toss, really. I've had rubs adhere quite well without any pre-rub treatments. If your rub doesn't stick well you can rub on a bit of salt first and let it sit a couple minutes while the salt draws out some moisture from the meat. I've used olive or canola oil when my rubs have no salt, but they typically will stick with natural moisture.