My experience with the /rolePropertiesFile option is that if you define your EntryPoint as shown in the example above (EntryPoint="myroleentrypoint.dll") in your role properties file, you will get the following exception when packing:

Unhandled Exception: System.ArgumentException: Illegal characters in path.
at System.IO.Path.CheckInvalidPathChars(String path)
at System.IO.Path.Combine(String path1, String path2)
at Microsoft.ServiceHosting.Tools.MSBuildTasks.CSPack.TryValidateEntryPoint(String outputDirectory)
at Microsoft.ServiceHosting.Tools.MSBuildTasks.CSPack.Execute()
at Microsoft.ServiceHosting.Tools.Packaging.Program.Main(String[] args)

This appears to be because of the quotation marks surrounding the entry point filename. Simply removing the quotation marks around this value in your role properties file (e.g. EntryPoint=myroleentrypoint.dll) avoids this issue.