Your customer wont listen to good advice - tricky. The Fast Flex is the best bet, we have used it in out of the ordinary situations and one offs and it was the only adhesive I would have trusted to do the job. I can also vouch for Ardex 7001 - using it for 2 display floors at H&G onto chipboard, laid loose and not retained on open sides, and only duck taped together; yes as bad as you'd want. Well it only had to last 4 days, (of a lot of footfall mind) but we were intending to do a good looking job - they were gardenset displays, fully dressed - The 7001 took it on its stride - there was no seam cracks or loose grout at all and then since we had to break it down at the end - crikey did that stuff take some getting apart!!
Given that it wouldnt last very long if this was a 'real' floor, however it was interesting doing it and seeing how well the gear worked.
Good luck. Make him sign off on it being done that way against your advice. These things can come back to bite you!
Colin