We had this problem in shul this past Shabbos. We had exactly ten people, with one Kohen. For whatever reason, the Kohen decided that he did not want the first aliyah. In order to preclude the gabbai from calling him up, he walked out of the shul into the hallway.
The problem, however, was that there were only ten people in shul. The gabbai called up a Yisrael, but now we were stuck. The person who was called up can't say Barchu, because we only had nine people. If the Kohen walks back in, even after Barchu, we have to set the Yisrael aside and call up the Kohen. What to do?
The rav thought about it for a moment and ruled in an interesting manner. He ruled that the Kohen was, in a manner of speaking, both present and not present at the same time. He was present for the purpose of having a minyan (he was still visible to us in the hallway), but yet, by getting up and walking out, he clearly demonstrated that he did not want the first aliyah and so, in that respect, he wasn't there. To do otherwise, he explained, would make two people unhappy - the Kohen, who clearly didn't want the aliyah, and the oleh, who would have to be set aside and stand by the bimah until the third aliyah. To cause two people unhappiness on Shabbos, the rav said, was something that he didn't want to do. So, the Yisrael said Barchu and the Kohen walked back in as the oleh was making the bracha.