Must one stand for a Rabbi present by video conference?
Rav Moshe Heinemann mentions in his sefer on Kibbud Av V'Eim (4:3):
Rebbetzin Shteif told Rav Moshe that whenever her husband spoke with him on the phone, he would stand up. Even though Rav Moshe didn’t know Rav Yonasan Shteif was honoring him by standing up during the phone call, Rav Yonasan Shteif still stood since Rav Moshe had the status of a gadol hador where you must stand up for as long as you see him walking (Shulchan Aruch YD 244:9) . However, if no one would know about the honor except for you, then I’m not sure if there would be an obligation to stand.
Perhaps on a video call where your Rebbi Muvhak or gadol hador would see you standing for them, there would be reason to stand out of respect. (Maybe this wouldn't apply for another Rabbi who would not technically be within four amos of you)
Update: Rav Moshe Lieber in The Fifth Commandment (III:E:note 39) quotes the Chayei Adam (67:7) saying that one does not need to stand up for his father if he is in a different domain, and the Birkei Yosef (240:1) equates the 'honor due a parent with that due a teacher'. While Rav Yitzchak Silver in Code of Jewish Conduct (p. 449) follows the same halacha not needing to stand for a Rav who is in a halachically different domain (on the bimah fenced off by a railing of 10 tefachim high), he also notes that the obligation of standing for a Rav within four amos is because "only then is it clear that we are getting up in his honor." Perhaps for the case of video conferencing, it would be noticeable you are standing for the Rabbi's honor even beyond four amos - so it may not be required since you are in a different domain, but it may be honorable to do so.