Hi, I attended a presentation this week, in Middlebury, (Addison County Bee Club) concerning the extended use, by Jack Rath. (Previous owner of Better Bee) Jack had used it last summer and the year before, with a special exemption. He had really good luck with it. With 90 hives he had a 86% winter success.
The trick to use this is to start it early in the Spring, while the mite level is very low. (Or after another treatment) This is a long term method, 60 to 75 days, and works fairly slowly. If there are a lot of mites it does not work as well. The bees have to walk on the pads to effect the mites. The Vt. Agriculture Dept. has a recipe for preparing the oxalic, glycerine and pads (VBA too I think). If you try it be sure to follow the directions, heating temperatures, and safety equipment, rubber type gloves, safety glasses etc.
According to Jack it does not seem to adversely effect the bees/queen like other treatments (Formic for example). If you try it make sure the mite count is very low. I tried it last summer, without knowing all the instructions, and placed the pads with a too high mite count and also not the proper amount of square inches The Swedish sponges are 8 X 7, split in half the two pads, 3 1/2 X 8 are placed between the two deep hive bodies.
Following my treatment I vaporized the hives, several times, with Oxalic Acid and so far so good this Winter. I suggest you look at Randy Olivers, Scientific Beekeeping, to get more info.