If I open Pidgin and then join a scheduled conference via a "meet.skype" link, the appearing chat window's Share my desktop option (and every other option under More...) is missing.
(It works automatically and initially with normal contacts.)

Once I open a personal chat with a random contact, and then tab back into the meeting's chat, the option appears.

After clicking on the issue, sharing the desktop works flawlessly, so this seems to be either a GUI bug or something SIPE not telling Pidgin to show this option or something.
The issue persists after the "Chat" window is closed and then reopened: the same workaround must be used, also if Pidgin is restarted.
Share my Desktop appears flawlessly for normal personal contacts.