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