T4 Scaffolder using the the module created by @ulfbjo [ http://pastebin.com/rjYdsuD7 ]

DerivingTypes.ps1

[T4Scaffolding.Scaffolder(Description = "Enter a description of Generator here")][CmdletBinding()]
param(
	[parameter(Mandatory = $true, Position = 0)][string]$ParentTypeName,  
    [string]$Project,
	[string]$CodeLanguage,
	[string[]]$TemplateFolders,
	[switch]$Force = $false
)

$parentType = Get-ProjectType $ParentTypeName
$parentNamespace = $parentType.Namespace.Name

$derivingTypes = @() 

# Get the namespaces that match the parent type
$namespaces = $parentType.ProjectItem.ContainingProject.CodeModel.CodeElements | Where-Object{ $parentNamespace.StartsWith($_.FullName) } 

$namespaces | ForEach {

	# find child namespaces matching the parent type
	$_.Members | Where-Object{ $parentNamespace.StartsWith($_.FullName) } | ForEach {
		
		# find classes in namespace
		$_.Members | Where-Object{ $_.Kind -eq 1 } | ForEach {
			$current = $_

			# check if class inherits from parent type
			$current.Bases | ForEach {
				if($_.FullName -eq $parentType.FullName){							
					$derivingTypes += $current		
				}
			}		
		}
	}
}

$derivingTypes | ForEach {
	Write-Host $_.Name
}