The only way a 70V distributed system would have any chance of functioning in this setting is if it is used solely for spoken word. Then it might work, so long as they are happy with the vocals sounding like the paging intercom of a grocery store. You can forget about any musical programme.
With only a rough concept of the room in question, I would say your center hung concept was the superior design. For general purpose vocal and music reinforcement, pair of good Community boxes will trump ANY commercial 70v 6 speaker array, hands down.
If there's serious money on the line, I'd suggest you bring in a local authority, whose expertise will not be in question. They will make short work of any wannabe designs. Another option is simply to crunch the numbers yourself, and then show the client on paper why a 70V system is poor design. Explain that it is impossible to even present a 'design' without listing the model and specs of the speakers in question, so that issues like output and coverage can be discussed in an objective fashion.
You could also do a direct demo: bring in a pair of good 70V speakers, and run them on the 36W the other designer thinks is adequate. Put a single decent pro cabinet up against that, and let the clients hear the difference. Then they might actually understand what they would be paying for in a set of real speakers, instead of some BG crap.
P.S. - I want a speaker with 180 X 120 dispersion!