As far as legality goes, I don't think there is any reason you can't limit what you petition the town to take over any way you want. But bear in mind that if the people on the "unaccepted" portion are not in agreement with being excluded, that could raise problems.
My biggest concern, however, is whether or not the Town is likely to actually accept the road. With the increasing costs of road maintenance and the strain that places on town budgets, many towns now have a policy of not accepting new roads unless they are first brought up to standard, as you mention. Other towns have a policy of not accepting ANY new roads, even if they are brought up to full town standard first.
To add even more uncertainty, if your town has the town meeting form of government, the acceptance has to be approved by a vote of the townspeople. If they think it will increase their taxes, there's a good chance they will vote against it, even if the Selectmen have led you to believe that it's just a formality once the road meets the required standard.
Your chances are slightly better if properties on your road contribute a substantial amount to the town tax base, (for example if there are many high-priced houses or many shorefront lots,) and you can argue that you are getting virtually nothing in return. I like to call it "mining taxes," as it gains the town a lot of money that it can spend elsewhere in town instead of putting it back into the road that generated it.
Bottom line - before putting a lot of money into improving the road, you need to carefully consider the chances of the road actually getting taken over by the Town. It might be more productive to try to track down the subdivider and demand that he finish what he started.