I'm not sure why you'd want to manage by FileGroup, but... What if you set up the same process that I outlined, but used this query to gather the data. I think it'd do what you want.

SELECT fg.GroupName

,SUM(f.size / 128.) AS TotalSizeInMB

,SUM(CAST(FILEPROPERTY(f.NAME, 'SpaceUsed') AS INT) / 128.) AS SpaceUsedInMB

,SUM(f.size / 128. - CAST(FILEPROPERTY(f.NAME, 'SpaceUsed') AS INT) / 128.) AS FreeSpaceInMB

FROM dbo.sysfiles f

JOIN [dbo].sysfilegroups fg

ON f.groupid = fg.groupid

GROUP BY fg.GroupName