Hi izzi81,
You are talking more about zoning rather than actual firmness (firmer under some parts of the body and softer under others) which can certainly have an affect on neutral spine alignment. All mattresses are a combination of firm (deeper layers) and soft (the thickness and softness of the upper layers). Outside of zoning, in some cases a firmer support core can help and in others thinner comfort layers can help. It all depends on how evenly you sink into a mattress. Post #4 here has some ideas along the lines you mentioned about adding a form of zoning in the context of helping a mattress that is sagging or too soft in one way or another (either support layers or comfort layers) that may be helpful.
Phoenix