If Michigan PHRF is similar to Midatlantic, they will tell you sail material is not penalized. They should also tell you that you then need to sail under the PHRF rating (144 from these messages), and take whatever adjustments are required by the bylaws, i.e., -3 for pole and -3 jib size.
The 144 Michigan PHRF rating makes sense to me - it nets you back to 138 when pole/jib are penalized, reflecting no penalty for material (although you should have a certificate that shows which configuration you're sailing). Midatlantic is, by their own words, "discouraging non-OD sailing" by making the PHRF base faster; i.e., 141, creating problems for J/30's with different inventories sailing in PHRF fleets - driving up the cost of ownership by requiring boats to buy both OD and PHRF sails. The class could ease this by relaxing the restriction on material (we all seem to agree it's not a performance penalty).
