I've gotten this command to work inside a script object:

Public Sub Main()

Shell("c:\documents and settings\sqladmin\desktop\plink -i g:\sshkeys\sqluser.id_rsa.ppk myuser@211.999.999.999 ""rm myfile_20120727.csv""")

Dts.TaskResult = Dts.Results.Success

End Sub

Not sure if it's a hack. Not sure if it'll still work when scheduled inside a job, but it's doing what I want for now. Thanks for the help gents!