My grandmother passed away on the Friday of Thanksgiving weekend and was not buried until Sunday. Should kaddish have been said already over Shabbat, even though aveilut does not begin until burial?
This question was addressed by Rav Moshe Feinstein in a teshuva written in 1981. In the teshuva, Rav Moshe notes that this is subject to a debate between the Taz and the Shach, with the former ruling that kaddish should be said and the latter (in the Nekudat HaKesef) writing that there is no need to say kaddish. The Shach explains his reason by noting that ther purpose of kaddish is to help the soul of the deceased be saved from gehinom, and since that is not even a possibility until burial, there is thus no reason for kaddish to be said.
Rav Moshe supplies a reason for the Taz's viewpoint that kaddish should be said. While there is no issue of gehinom until burial, there is still the issue of the deceased being judged in the heavenly court, and kaddish can certainly serve as a merit in that judgement even before the body has been interred. Rav Moshe feels that even the Shach would see merit in this rationale.
While the Avodat HaGershuni comes out squarely against saying kaddish at all before burial, Rav Moshe notes that the Gesher HaChayim of Rav Yechiel Michel Tuketchinsky (the authoritative source for the laws of mourning) sides with the Taz, and Rav Moshe seems inclined to do so as well.