Welcome back to mapmaking DiM. If you need any ideas or tips, I've made my own steampunk map present in the sig. The idea itself is sound, and since R&C got an exception to go 6, if you justify it well you should be good.
As cairns said, <overrides> tags make that a piece of cake.
One major mapmaking change is that we now have the authority to make a map supersize if its concept justifies the extra space. Smalls can go 1000x800 and larges can go 1400x1200. So if you could do 8 players with the larger sizes, thats a possibility. I get the impression that you specifically want the map to be 4p, so we'll have to wait and see the full concept.
Process-wise, it's fairly similar to the way it was in 2008-09. Now that you've got a functional draft up I'll be moving Age of Steam over to the Drafting Room after I post this. After you get a Draft stamp it's the same gameplay-graphics-XML you remember. The major difference is that maps quench to Beta status first, leaving you liable to making changes as balance requires, and when all those concerns are handled (~1 month later), quenched for good.