Are there any known halacha- or responsa- sources that deal with the permitting or prohibiting of trimming eyebrows during Sefira? I'm wondering if eyebrows fall into the "regular" category of haircut prohibition during Sefira or if perhaps eyebrow trimming is somehow halachically different from a regular haircut and therefore permitted.
Piskei Teshuvos 493:7 cites the Sefer "Bein Pesach Le'Shavuos" (9:11) in the name of R' Shlomo Zalman Auerbach and R' Shmuel Vozner that plucking eyebrows or eyelashes during Sefiras Haomer is permitted although it is done for beatification, since this is not characterized as "תספורת" (cutting).
Rabbi Gavriel Zinner also brings this ruling in Nitei Gavriel (Pesach vol. 3 49:4, see Hilchos Bein Hametzarim 19:4) in the above Rabbis' names, but notes that R' Moshe Feinstein did not permit mourners to shave their eyebrows during first 30 days of mourning.