The gemara says clearly that a temporary mum doesn't qualify as a mum at all to permit the shechting of a bechor outside the mikdash, or for redeeming pesulei hamukdashin. However, it qualifies as a mum to prevent the animal from being brought as a korban until the mum heals. It isn't clear if a temporary mum would invalidate a kohein from doing the avoda even temporarily.
The Chachamim in the braisa say that a kohein cannot become a nirtzah since it will require the making of a mum and invalidate him from doing avoda. From this the gemara proves that the piercing is done in the cartilage part of the ear, rather than the fatty part, since in the fatty part it wouldn't make a mum. Rashi explains that a hole in the fatty part would heal and therefore not be considered a mum. Rashi implies that since the mum will heal, it isn't considered a mum at all to invalidate the kohein from doing the avoda, even temporarily. The Maharit Algazi points out that the third answer of Tosafos (d.h. ka'an) that a temporary mum would invalidate the kohein temporarily seems to be against Rashi.