If you let it sit in the sauce overnight then it's no longer a sauce, it's a marinade.
That's fine, but marinating is something you generally do with tough, cheap, and/or dry cuts of meat such as chicken breasts or top rounds (beef). For a full chicken, especially the wings, drumsticks and thighs, a marinade is entirely unnecessary and in my humble opinion simply dilutes the deliciously moist interior and often makes the whole thing soggy and depressing.
Generally the goal (or at least my goal and that of the majority of restaurants I've been to) for chicken wings or even a traditional roasted whole chicken is a nice crispy, golden-brown, well-seasoned skin with juicy meat on the inside.
You can't get that if you douse it in sauce before grilling it. The water in the sauce will inhibit the Maillard reaction that causes browning (and crisping), and as Mike points out, the sauce may even burn. In fact, it almost certainly will burn; most store-bought BBQ sauces only last a few minutes exposed to direct heat.
With that in mind, the best time to sauce a grilled (BBQ) chicken is after it's cooked, or more precisely, when it's nearly cooked. Go ahead and season it liberally and coat it in butter or oil beforehand, but wait until it is no more than a few minutes away from being done before you sauce it. You're not trying to cook the sauce, and it takes no more than a few minutes to get the sauce to bind to the skin - i.e. a few coats with 30-60 seconds each to reduce and form a sort of glaze.
This is true for almost any meat and any cooking method as long as you're not marinating - BBQ chicken, fried or convection-baked wings, smoked or oven-braised ribs - you almost always want to sauce it right at the end. If your technique and ingredients are good then the meat, not the sauce, should be your main attraction.
I'd make a simple raspberry coulis by blending fresh raspberries with a little water in a blender, with perhaps a touch of sugar depending on their natural sweetness, then passing the results through a sieve to remove the pips. No need to get fancy, the cheesecake is the main event!
Best Answer
Put simply (and based on the previous answer), no, it's not.
From Flickr
Recently, images have finally appeared of the real Szechwan Sauce packets on Flickr, as well as a video on YouTube, both of which clearly show the original ingredients list as:
And following the "one day limited release," photos of the modern recipe have been posted online, showing the ingredients as:
Ignoring the ubiquitous water, salt, and HCFS, we're still left with a number of "missing ingredients" that are present in the BBQ (tomato paste, grape vinegar, dried chili peppers) and the S&S (apricot/peach puree, dried chili peppers). The first of each of these missing ingredients contributes the primary flavor of the sauce. Meanwhile, both the BBQ and S&S sauces are missing Szechwan's ginger and (roasted) sesame seed oil, as well as the larger amount of vinegar, the key flavor components for that sauce.
Based purely on the ingredient lists for all three sauces, even taking into account what may be a "modernized" recipe for any of the three, Szechwan sauce cannot be a blend of BBQ and Sweet and Sour sauces.