Question: Is a rabbi needed for a baby naming ceremony?
Answer: You asked about doing a baby naming while your rabbi is on sabbatical. I hope that your rabbi has left some instructions with the congregation about the proper way to handle life-cycle rituals during his or her absence. If not, you should try to reach your rabbi or another rabbi in the area to determine the best course.
In general, rabbis are not required for Jewish rituals. Rabbis are teachers and authorities in matters of Jewish tradition, not professional ritual leaders.
The role of rabbi, in this way, differs from the role of clergy in other religions. There is nothing about a rabbi that makes him or her "more effective" in leading rituals than a knowledgeable and capable lay-person. In principle, there would be nothing wrong with such a Jew conducting a baby naming service.
People often assume that a rabbi is needed to conduct funeral and wedding services, but this is not the case in Jewish tradition. Rabbis, like other members of the clergy, are given authority by the government to solemnize legal weddings. Many Jewish communities and rabbinic organizations allow only rabbis to officiate at Jewish weddings for that and other reasons, but a competent lay-person, granted appropriate civil authority, also could serve as a m'sader kiddushin (wedding officiant).
In the case of a conversion, which requires a beit din (rabbinic court), the leadership of rabbis is required. This is a case in which a rabbi actually is acting in the role of judge, not as a ritual officiant. Of course, only rabbis are permitted to ordain other people as rabbis.
One more thing ... if this baby naming involves a b'rit milah (bris), perhaps you could let a competent mohel wield the scalpel?