A new torah arrived with only 6 letters open for the siyum. They want to honor about 20 people. Can we scrape off some letters & let the people fill them in?
A Sefer Torah with open (hollow) letters at the end is considered a kosher Sefer Torah (see Divrei Malkiel 4:90; Os Chaim 32:4; Shoel U-Meishiv 3:1:390; Tzur Yaakov 155; Shraga Ha-Meil 3:117, among others; see also Beis Yitzchak Vol. 2, Kuntres Acharon 20; Shevet Halevi 3:96:3).
Therefore, letters should not be scraped off, for doing so is only permitted for the purpose of fixing the Sefer Torah, and not for other purposes. This ruling is given explicitly by Shut Shraga Hameil (7:30).
See also Adnei Paz, Vol. 4, no. 141, who writes at length on the matter, and cites many authorities who write that one must refrain from erasing any letters of a Sefer Torah (not for the purpose of fixing the Sefer), even if the Sefer Torah is not entirely complete, and the more so when the Sefer is complete.