Well, I've always wondered why MS couldn't just add a darn column with total space, but it was not to be.
The only way, I know of, is to use other methods outside SQL Server, such as a VB or ASP script, that uses the FSO (file system object).
Also, you can use Srvinfo.exe which is in the W2K Resource kit. It does output a lot of info about the server,services, uptime, etc., but also all the disk volumes with total, free, and used space.
You can execute this via xp_cmdshell, and perhaps insert it into a table, and then get just the disk info. Just an idea that hopefully you can work with.
HTH