Well, I'll throw my hat in to the mix and also state that I have only ever pronounced it as 'plaad'! I can't say that whether it is historical or not matters to how I will continue to pronounce it.

Thank you to those who have shed some light on what the original items in question. I think that they are cooler than a fly plaid if people still go in for that sort of thing, certainly more practical.

Best,
Adam