You could use the Lobster Pot Helm from the cav actually Jack, great idea btw! I forgot the cavalry came with these as I don't own a set. I also didn't realise they came with floppy hats, so even more scope for conversions.
You could then put together a Forlorn Hope. Foot slogging Troopers, or just more armoured Infantry to act as a storming party.
I think the cav come with Cuirass bodies, mixed the the 'legs' of the WSS Infantry, so even more possibilities there, as with the Pistol arms from the cavalry sets! Actually didn't someone on here do some ECW era cavalry with the WSS figs?
It's definately more of a possibility that I had originally anticipated, so I hope my earlier comments didn't put you off.
One thing to remember, there was very little uniformity, especially in the early days of the ECW, so there's alot of scope for experimentation. You could even have more smartly dressed Irish Lords/Nobles to lead rabble against the NMA.....though I have to admit I personally am in league with Cromwell...ahem...