Eureka!

The trick is in the path. LabFrame does not have a .font property, but its subwidget of Label does:

use warnings;
use strict;
use Tk;

my $mw = MainWindow->new();
$mw->geometry('200x200');

$mw->optionAdd('*font', 'helvr12');
$mw->optionAdd('*LabFrame.label.font', 'helvb12');

my $lab_frm = $mw->LabFrame(-label => 'LabFrame Test')->pack;
$lab_frm->Label(-text => 'Label Test')->pack;

print STDERR $lab_frm->Subwidget('label')->PathName, "\n";

MainLoop;
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Update: Another interesting finding. This does not work with LabEntry because, from what I can tell, it does not advertise its Label subwidget. This can, however, be worked around by changing both subwidgets, then reverting Entry:

$mw->optionAdd('*LabEntry*font', 'helvb12');
$mw->optionAdd('*LabEntry.entry.font', 'helvr12');
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