You're on the right track with the MSBuild Extension Pack.
I find the easiest way to conditionally generate the assembly details at build time is to add an "AssemblyVersion" target directly to my .csproj file(s) that require an updated AssemblyInfo file. You can add the target directly to each csproj file that requires an updated AssemblyInfo file, or as I prefer to do it, create a a custom targets file with the AssemblyVersion target and have each csproj file include your custom targets file.
Either way you likely want to use the MSBuild Extension Pack or the MSBuild Community Tasks to use their respective AssemblyInfo task.
Here's some code from our build scripts:
<!-- Import the AssemblyInfo task -->
<Import Project="$(MSBuildCommunityTasksPath)\MSBuild.Community.Tasks.Targets"/>
<!-- Overriding the Microsoft.CSharp.targets target dependency chain -->
<!-- Call our custom AssemblyVersion target before build, even from VS -->
<PropertyGroup>
<BuildDependsOn>
AssemblyVersion;
$(BuildDependsOn)
</BuildDependsOn>
</PropertyGroup>
<ItemGroup>
<AssemblyVersionFiles Include="$(MSBuildProjectDirectory)\Properties\AssemblyInfo.cs"/>
</ItemGroup>
<Target Name="AssemblyVersion"
Inputs="@(AssemblyVersionFiles)"
Outputs="UpdatedAssemblyVersionFiles">
<Attrib Files="%(AssemblyVersionFiles.FullPath)"
Normal="true"/>
<AssemblyInfo
CodeLanguage="CS"
OutputFile="%(AssemblyVersionFiles.FullPath)"
AssemblyCompany="$(CompanyName)"
AssemblyCopyright="Copyright $(CompanyName), All rights reserved."
AssemblyVersion="$(Version)"
AssemblyFileVersion="$(Version)">
<Output TaskParameter="OutputFile"
ItemName="UpdatedAssemblyVersionFiles"/>
</AssemblyInfo>
</Target>