I can do it, but I'd rather have someone from Harman do it so they understand how and I don't have to keep up with future releases or if they decide to change their download URLs someday. If they can commit to updating this file, then they can do it as part of their releases and folks won't have to keep asking other Flex committers to update the file.
You can specify these on the command line, or create a local.properties file in the same folder as installer.xml
The first 3 are concatenated to form the URL for the download. The last is the MD5 of the download. Since you'll probably have to download the SDK to compute its checksum, you can probably just use file:// as the server variable to point to the download and not have to re-download the large SDKs.
You can compute the MD5 using the build.xml in the SDK:
ant md5 -Dfile=<path to file>