Strictly speaking, a carbon mast on a 5.5 sloop means you're not sailing class legal configuration, so the RC can penalize you anyway they feel is fair.
If you want get a Portsmouth number, what I would recommend is to rig it that way, race a few races with the understanding you are not racing for a trophy, only racing generate times for Darlene so she can generate a provisional number. Make sure the RC & regatta chair know what you are doing, announces it to the other competitors, and submits the data to Darlene.
An alternative is to buy a 5,5 uni carbon mast, and race it as a uni.