I figured out a way.  I saw a snippet of code in one of the videos on the topic, and it gave me enough clues to write a callback.  Here is the radio button callback in case it helps anyone!
rbc = get_param(gcbh,'HS_Select');
h = get_param(gcbh,'MaskObject').getDialogControl('ContainerCustom');
switch rbc
    case 'Sink Vert'
        r = '1.6';
        t = '1800';
        h.Enabled = 'off';
        set_param(gcbh,'rthedit',r);
        set_param(gcbh,'tauedit',t);
    case 'Sink Horiz'
        r = '1.3';
        t = '1300';
        h.Enabled = 'off';
        set_param(gcbh,'rthedit',r);
        set_param(gcbh,'tauedit',t);   
    case 'Ideal Heat Sink'
        r = '1e-9';
        t = '1e-9';
        h.Enabled = 'off';
        set_param(gcbh,'rthedit',r);
        set_param(gcbh,'tauedit',t);
    case 'Undersized Heat Sink'
        r = '7';
        t = '30';
        h.Enabled = 'off';
        set_param(gcbh,'rthedit',r);
        set_param(gcbh,'tauedit',t);
    case 'Custom'
        h.Enabled = 'on';
end



