Mimic The Rowcount Transform's Variables Dropdown?

I'm trying to mimic the rowcount transform's custom property called VariableName in a custom pipeline transform, with the same drop-down and list of variables in the default property grid as that of "RowCount".

Should a UITypeEditor or a TypeConverter be used for such a custom property? More importantly, in either case, how do you get from the UITypeEditor or TypeConverter to the TaskHost which contains the Variables to enumerate?

I'm not that familiar with Windows Forms, but it appears in both cases (editor or type converter), an ITypeDescriptorContext parameter named "context" is passed, but I'm certain how to get from A to B, that is, from the context variable to the TaskHost.

P.S. There's no need to add a new variable, just select from those that already exist.