I posted an update, but it seemed to be removed through the forum "upgrade". Anyhow, it turns out the owner of the restaurant found out from the city, that he didn't need to have a patch panel installed after all, so I haven't had to return back to the three phase receptacle job site.
I am fairly confident however, that I could have made an pigtail to use my welding machine off that 3 phase receptacle, though. Maybe I'll have another opportunity where I need to adapt from 3 phase sometime.
Agree, single phase xfmr is a proper idea, trying to make single phase 240 off of a delta or Y is asking for an exciting life, perhaps even a short one.
Actually, ground won't but their will be flow on the neutral which attaches to the same point as the ground, but ground, as you say, should never be used as a current carry conductor, only for fault current.
Around here 240 D is popular, particularly with center pivot irrigation.
Any two legs of a 240 D can be made to work for 240 single phase, but it may run unbalanced (as I have been told). Its best to use the two legs measuring 115 V to ground (nominal) and the third leg will measure 200+ to ground. But between any two hots, it will be 240.