Seems to me its loophole. Peer-pressure can be very difficult in an inter-church Communion gathering. The Church of England is a part of Porvoo, including indirectly all it’s subsidiaries. Do you mean that if the Church of England invites both the Church of South India and one of the Porvoo Churches - both of which are in ‘full communion’, that one clergyman or bishop would refuse to attend due to the possibility of a woman-priest/woman-bishop? More likely they will all attend and

give some excuse later, if asked.
Good example, the Marthoma Syrian Church does not have or (currently) recognize female clergy (although it seems more and more likely that they will), but is in full communion with the CoE. They invited the Rowan Williams to India, he brought along some female clergy in his entourage. Are they going to tell their guests they are not allowed to vest? If they are vested, will they be asked to de-vest and stand with the congregation? The YouTube video on this particular occasion shows neither, they just ignored the glaring inconsistency.
On the reverse side, when the CoE invites all their sister communion partners along for the installation of the Archbp of Canterbury or some other prominent See, will most refuse out of the concern a female cleric may join in? I doubt it.