Nancy does not have a way to modify model after the route has been executed. The only option is to modify the serialized form. With this add-in it becomes possible to add custom processing of models once the route has been executed but before the result has been serialized.
Install-Package Nancy.ModelPostprocess.Fody -Version 0.1.8
dotnet add package Nancy.ModelPostprocess.Fody --version 0.1.8
<PackageReference Include="Nancy.ModelPostprocess.Fody" Version="0.1.8" />
paket add Nancy.ModelPostprocess.Fody --version 0.1.8