Chris, there are a bunch of ways to go with roadbed and all have strong and weak points, it's pretty much a matter of preferance. Since you are spiking, homasote and soft pine hold spikes best. Many swear cork holds spikes fine, I'm not one of them. Perhaps those who ballast after spiking have better luck with spikes being held in cork due to the glue used. I ballast prior to rail laying so lose that advantage. If you want a shortline with modest height roadbed, a good way to go would be homasote on plywood, as you mentioned, with N scale cork roadbed. When you align the sloped edges of the N scale roadbed at the proper spacing for HO, you'll have a space in the middle. You can use a 3rd strip of roadbed cut to fit, or if you have a hot wire foam cutter, cut foam to fit. The foam won't hold spikes worth spit, but the spikes ought to line up over the cork. The N scale cork is thin and the spikes will go thru into the homasote. Except for those places where you need to spike, you could fill in that space in the N scale roadbed with spackle or similar material. Run a knife over the top of the roadbed to level.
Rather than deal with getting 4x8 sheets of homasote home and then cutting it, I buy Homabed (California roadbed Co. , I think) which is homasote made into roadbed similar to cork. I think its a bit higher than cork, perhaps 1/4". I don't know because I don't worry about it. When I want the roadbed to be less high in comparison to the surrounding land, I fill in that land, usually using a mixture of sawdust, earth colored latex paint and carpenters glue. This stays soft enough to plant weeds and such for 12 hours or so. And its cheap!
