Jim, the rules, as presently written, do not allow such a local rule, it is contrary to the rules themselves. Rule 33-8 governs local rules and contains 45 decisions on what is and what is not allowed for local rules. A careful reading of that rule and 33-7 makes it clear such a local rule is not permitted.
TEPaul, et al. - The Committee did not have the right to waive the penalty of disqualification in this case, there is a decision - d 33-7/4.5 - specifically states a DQ penalty cannot be waived or modified for failure to include a penalty.
Regarding the status of Andy McPhee, he is one of two Chief Referees employed by the European Tour (the other being John Paramour). He has final authority at European Tour events in rules matters. He would be the final arbitrator for issues, such as this one, at the tournament site.
BTW, he and JP are world class officials, highly respected by the players and other officials. If there was a way for him to avoid imposing a penalty of DQ on Harrington, he would not only have know of it, he would have used it.
Geoff Shackleford has some interesting comments from McPhee, Mike Davis from the USGA and Grant Moir of the R&A on this issue.