Brin - what Clarkey is suggesting is exactly what we used to do in the olden days. Don't you remember them?

I'd play it in parallel with the actual FA Cup (e.g. on the Saturdays/weekends when the actual FA Cup games are played). The only complication we used to have is when we don't have exactly 16, 32...etc. players. What we used to do then is give a bye in the first round to the highest placed in the league, a bit like Prem/Champ get a bye in the real FA Cup. So for example if there are 22 players in the league you'd need to get it down to 16. So 6 matches would be played between the lowest 12 players in the league and the top 10 in the league would get the bye. That would then give you 16 for the next round.