When you purchase the software and download the installer, it comes in .zip format (170MB). Extract the .zip file then double-click the vRD2008SP1.exe (33MB) in the Installation directory. The other files are the version with SQL Express and documentation.
When you run the .exe, after specifying the language, the .msi is extracted. Check in %temp% for the newest .msi file and copy that somewhere before you click cancel. The .msi file is only about 8MB. You can then create a transform based on that.
With regards to licensing, you get a .xml file upon purchase that needs to be imported on a per-user basis. The information appears to be saved to [HKEY_CURRENT_USER\Software\visionapp\vRD2008]
"CoreLicense" = ...
However, exporting the .reg key and reimporting it on a test machine didn't fully license the product. We've only got 6 named admins so we're going to import the file for each named admin.