How about a non legal answer.

You could tell the FSDO about the idea, tell them you really don't want to
do it, and ask, "Is there any possibility that sport plane rules will get
modified so that planes can be modified back into LSA eligibility in the
future?"

The only correct answer to this is Yes.

Now, go to your pushy customer, and advise he wait, since the FSDO said that
the rules could change and allow a simpler solution.

Lot's of changes are coming as these questions start getting raised, and I
suspect your customer is much more likely to get a positive solution in 6
months than now.