Where alcohol policy meets the 3 martini lunch?

The real question is, why are you trying to impose a policy that doesn't make business sense?

Easy: "... excluding beverages purchased or furnished by [your company]." After all, the executives are most likely using the company provided AMEX, while Joe cubicle is paying out of pocket.

Honestly, what most companies do is have a blanket policy and then the salespeople and executives just violate it on their own. Everyone looks the other way most of the time, although if they were to do something stupid — like a DUI in a company vehicle on the way back from a business lunch, or run someone over with a golf cart at the country club — then they'd have created convenient grounds for disposing of themselves. The de jure policy is "don't drink"; the de facto policy is "if you drink off-site you're on your own." The only other even reasonably fair policy would be to tie it into the operation of heavy equipment, but a factory worker could pretty reasonably point out that if the executive is driving back from a business lunch in a car, they're operating heavy equipment that probably has more restrictions surrounding its use in connection with alcohol (and is arguably more dangerous to more people) than the die press or forklift or whatever they're operating in the factory. Which is why most places that I'm aware of that have drug-free/alcohol-free workplace policies have them apply (at least in theory) to everyone.

I'm guessing that business lunches etc would take place off-site, whereas the factory work would always be on-site - why don't you just say no alcohol on site? Or no alcohol around machinery etc? I don't really see much of a problem in the first place (it's expected that alcohol is not allowed in a factory workplace, and it's semi-expected that business lunches include alcohol), but if you really need to make a very general policy, the location clause should work for that.

An explicit policy authorizing consumption of alcohol while performing company business, in any setting, is a pretty direct track to liability for any resulting injuries/damages. The lack of such a writing doesn't eliminate the possibility of liability, but beware of endorsing consumption.

"Exceptions to this policy will be approved by..."

I would tread lightly trying to reconcile this yourself - have you checked with your peers in the local HR community or with SHRM etc? There will be examples of what you need already built, legally vetted and HR approved.

I usually see it as "no alcohol consumed on the premises," occasionally modified to "no alcohol consumed on the premises except for official company events" (so you can have beer at a company-sponsored bbq, as well as wine for an on-site executive dinner), and a very important additional rule that boils down to "don't work if you're impaired". which has the benefit of including prescription and OTC drugs as well as alcohol and recreational illegal drugs.

I think Kadin2048 has it right. Also, I'm not sure of the legalese here but maybe a slightly different wording would help, ie "impaired" instead of "under the influence of"? Having a drink or two with a client over a long lunch won't "impair" the exec, but I would assume you don't want them getting sloshed and then coming back into the office, regardless of their rank in the hierarchy.