The solution is to first extract the contents of the .NET 4.5 installer executable using the command: dotnetfx45_full_x86_x64.exe /x
Then I can use a Custom Action in my UI sequence to invoke the .MSI version of the .NET installer (without it's setup.exe wrapper!)
msiexec.exe /i netfx_Full_x64.msi EXTUI=1 /l*v “Install.log”
This fully resolves my original question.