You can try the hypenated version of -SelBlockInstanceNamed -> SelectBlockToMatch to pick source block instances and select all of the instances of these blocks. Works on multiple block definitions.
(many commands when used with “-” before run in command-line mode only and sometimes in V5 have options not available in regular command mode. This is one of them.
I know this is an old thread but here is a simple script to select similar blocks, using the method Jarek suggested with pre-select accepted :
Sub SelectSimilar()
Dim strObject, StrName
strObject = Rhino.GetObject("Select block",, 1)
If Rhino.IsBlockInstance(strObject) Then
StrName = Chr(34) & Rhino.BlockInstanceName(strObject) & Chr(34)
Rhino.Command "-SelBlockInstanceNamed " & StrName
Else
Rhino.Print("No Block Selected")
End If
End Sub