No way to apply coupon for renewal promo

We want to offer a renewal promo using a coupon code, but there is no way to apply a coupon when the member clicks the renew now button.

I see that the payments modal window is handled by two different functions in class_white_label_themes.php ( TOOLBOX and RENEW_TOOLBOX). They are constructed differently and there is no option for coupon codes in RENEW_TOOLBOX.

Is there a reason that it was constructed this way? We really want to be able to offer coupon codes to be used for both new purchases and renewals.

Thank you!