I would suggest the same as games are now, so a torrent file per layout (the .gam, .dat etc). and another torrent for the ROMs.
It would need a dedicated person/small group to go through the layouts and host them to begin with.
With amber being aimed at the higher resolutions, some layouts could break 100MB with different JP/Decal options and high res artwork. Not the sort of size that the forums can host without major bandwidth costs.
The issue that I have is that we - in my opinion - have a large proportion of 'leechers' here. Don't take me wrong, the posting community is fine and people have a right to leech, I'd just be a little concerned that you'd end up with a very small base of users willing to seed the full 'works', and not enough seeders of everything.
Might not be an issue though - like I said I was going to suggest BT myself as a way of sharing content.
I did say above that I wasn't sure about individual files e.t.c, then I remembered that when building up some abandonwarez and what-not, many of them were HUGE torrents, but because the files were archived individually within the torrent, you could pick and choose what you wanted without having to download the complete set - that might be an option?
I'd love to say I'd be willing to seed but I don't pay for the internet here. I am looking at moving to an FTTC connection but would need to look into usage constraints - Sky on normal their Unlimited package have been very lenient with me, but I'd hate to get a faster connection and then be hammered for potentially uploading TBs of data over a month.
BT does seem to be the way forward though. 'Upload' sites are too problematic and the forum costs do seem too high, and BT seems very easy to implement. I'd do what I could to help (within reason!).