I ended up creating a new recipe in a new cookbook to handle installing .Net Framework 3.5 on Windows Server 2012.
I was able to find the needed files (since I don't have an install disk, I'm on Azure) by dowloading an eval ISO from here.
After mounting the ISO I zipped the entire ..\Sources\sxs folder.
my one execute command looks like this:
execute "install_net35" do
command "C:/Windows/sysnative/dism.exe /online /enable-feature /all /featurename:NetFx3 /norestart /limitaccess /source:C:/tempdirectory/sxs"
...where "C:/tempdirectory/sxs" is the unzipped file.
I also wrapped the execution in a check for Windows Server 2012:
if win_version.windows_server_2012? || win_version.windows_server_2012_r2?
..and do a check to see if the framework is already installed (to ensure idempotence):
unless File.exists?('C:/Windows/Microsoft.NET/Framework/v3.5')