Well look at Boost::Preprocessor, if there isn't a way to do this with that library, there is no way to do it period. There is BOOST_PP_STRINGIFY which takes a non-literal string like just abc and turns it into a literal string "abc". There might be other things of that flavour and that would somehow achieve what you desire.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...