Yesterday, we mentioned the Radbaz, who said that even according to the Rambam, the Minhag (custom) of Mitzrayim (Egypt)—not to wash the hands for Birkat Kohanim (priestly blessing)—does not apply to Musaf (additional prayer service). There are several reasons for this. The simplest is that there is too much Hefsek (interruption)—too much Hefsek between the morning Netilat Yadayim (wash the hands) and Musaf. How can you be sure that the person did not touch something that would make the hands impure? Therefore, you cannot rely on the Rambam for Musaf. So even according to the Egyptian Minhag, this applies only to Birkat Kohanim of Shaharit (morning prayer); it does not apply to Birkat Kohanim of Musaf.
Among the Mefarshim (commentators), the Magen Avraham agrees with the Radbaz, and the Knesset HaGedolah explains the following. It is not Musaf per se. Here's what I mean. Let's say a Kohen (priest), according to the Rambam, washed his hands for Birkat Kohanim of Shaharit. Can he rely on the Netilat Yadayim of Shaharit to cover Musaf? That's a different story. Maybe he could.
According to Rashi, you have to wash every time you are going to do Birkat Kohanim. It's like eating bread: you don't say, “I didn't do Netilat Yadayim this morning for breakfast, so I don't have to do it at night.” Although the Shulchan Aruch says that if you had Kavanah (intention) and did not have a Hesech HaDa'at (distraction of the mind), it could work, without going into that—because these are not the halachot (laws) we are discussing right now—you would have to do it each time. According to the Rambam, one could cover the next one. Theoretically, if you did it for Shaharit, were thinking, and were making sure that your hands were clean, that could cover Musaf.
But would that work from the morning Netilat Yadayim for Shaharit all the way to Musaf? That's when the Knesset HaGedolah, the Magen Avraham, and the Radbaz would say, “No, that doesn't work.” Even according to the Rambam, it's too far, especially since there is a certain Hefsek between the Netilat Yadayim and the Birkat Kohanim of 22 Amot (cubits) that we discussed a few days ago.
Bottom line, just to conclude this discussion, Chacham Ovadia tells you the level of Lechatchila (from the outset). Lechatchila, and especially according to the Shulchan Aruch, who was posek (halachic decisor) in accordance with Rashi's Halachah (Jewish law), a person has to do Netilat Yadayim every time he is going to do Birkat Kohanim. Even if you already did the Netilat Yadayim of the morning, Lechatchila you should do it for Shaharit and for Musaf. But b'diavad (after the fact), if a person wants to be somech (rely on) the Rambam, that's okay, but just the one before. Meaning, if you did Netilat Yadayim in the morning and your hands are clean, and you made sure that they were clean, then you could cover Shaharit. But that's not enough to cover Musaf, so it may be necessary to do it for Musaf. Or, if a person did Netilat Yadayim in the morning and for the first Birkat Kohanim of Shaharit, then for Musaf, if he was careful with his hands, he could already be covered for that. But Lechatchila, Chacham Ovadia is clear that one has to do all the Netilat Yadayim for Shaharit and for Musaf.