Finally found an explanation: http://blogs.msdn.com/b/sqlforum/archive/2011/04/28/walkthrough-how-to-get-distinct-values-of-a-column-of-a-sharepoint-list-using-sql-server-reporting-services.aspx
Essentially, this has you create one parameter to serve as a sorta jury-rigged dataset based on a piece of custom VB code. That first parameter is then used to populate a second, multi-valued parameter, which contains all unique values.
The VB code:
Public Shared Function RemoveDuplicates(parameter As Parameter) As String()
Dim items As Object() = parameter.Value
System.Array.Sort(items)
Dim k As Integer = 0
For i As Integer = 0 To items.Length - 1
If i > 0 AndAlso items(i).Equals(items(i - 1)) Then
Continue For
End If
items(k) = items(i)
k += 1
Next
Dim unique As [String]() = New [String](k - 1) {}
System.Array.Copy(items, 0, unique, 0, k)
Return unique
End Function
NB: When you set up the dataset in this process, make sure to alter the query to reflect your own data source; don't just copy/paste blindly.