I just wanted to report that I was able to get NeonMika.Webserver working on my N+2 with framework 4.3.1 with ease. There are 2 things that need to be done.
1) For each project, right-click>Properties>Application and update the Target framework to ".NET Micro Framework 4.3"
2) For each project, expand the References folder, remove any reference to Microsoft.SPOT.* and SecretLabs.NETMF.* and re-add the 4.3.1 versions of each
Took me all of 5 minutes and fired right up!