This is fairly well-known among those that follow along on Flyertalk or the Saverocity forums, but I recently discovered the following hack that allows you to work around the restriction of these vouchers not being able to be applied to the taxes and fees of a flight:
Basically, what you need to do is to make a dummy reservation with the voucher (of value < voucher) and then immediately cancel that reservation. The voucher then gets converted into “Travel Funds” associated with the initial booking. These can then be applied to any portion of the booking you actually want to make (including the taxes and fees), and since you can add a LUV voucher as an additional form of payment, you can consume the rest of the voucher as part of your new booking.
Hope this helps someone!