It is allowable to wipe over any wrapper or diseased or injured bodily part. There are many hadith on this point, and although they are all weak, their many chains strengthen each other, making them valid to talk about. One hadith, that of Jabir (quoted earlier), relates a story about a man who was on a journey and suffered an injury. While he slept, he had a wet dream, after which he asked his companions if he could perform tayammum. They said he could not, so he made ghusl and died because of it. When that was mentioned to the Prophet, he said "They killed him, may Allah kill them. Do you not ask about what you do not know? ... It would have been enough for him to perform tayammum and drop a little water over his wound or else wipe it, then to wipe it and wash the rest of the body." This is related by Abu Dawud, Ibn Majah, ad-Daraqutni and Ibn as-Sakin, who classified it as sahih. Ibn 'Umar used to do this.
In fact, it is obligatory to wipe over such casts or wrappers in ablution or ghusl instead of washing the injured parts. This must be done even if he has to heat the water. But, if he believes that this would harm the diseased or injured part, or that his condition may worsen, or that his pain would increase, he may wipe the injured part with water. If he fears that this would also be harmful, he should wrap it and then gently wipe over it. It is not necessary for him to be in a state of purity while applying the cast or wrapper to be wiped. There is also no time limit for such wipings, for he can do so as long as his condition lasts. Removing the wrapper or cast nullifies the wiping, as does the final cure.