What is the Islamic rule regarding holding funeral prayer for an absent diseased led by an Imam through a live broadcast to avoid Coronavirus?
Answering your question, the European Council for Fatwa and Research, states:
All types of congregational prayers, be it one of the prescribed prayers or a funeral prayer for an absent diseased, can only be conducted when the congregation and the Imam are in the same place. So that if the Imam is unable to complete the prayer due to one reason or another a member from his congregation would step forward and substitute him in completing the prayer.
Since this is currently unavailable, then there is no harm if families offer their funeral prayers for the absent diseased while led by an Imam sharing the same place with them.
As for the prayers led through a live broadcast by an Imam based in a place different to that of his congregation, they are invalid, since the conditions stipulated for following an Imam in Friday prayers, funeral prayers and other congregational prayers are not fulfilled.
Funeral prayer for an absent diseased is usually offered when the body is not there in the front. The live broadcast does not change the nature of such a prayer. Hence, it is much safer to offer it in its commonly recognized way.
Offering funeral prayer in the way described in the question involves a number of mistakes, e.g. the congregation might be in a location ahead to the Imam’s.
In a number of situations, e.g. a problem in the receiver, electrical disruption, etc. following the Imam becomes inevitably unviable.
People may also go to the cemetery, when allowed, to offer their funeral prayer near the grave. Ibn Abbas reported that the Prophet Muhammad (peace be upon him) went to a grave which had been newly prepared and prayed over it, and they also prayed who were behind him and he recited four takbirs. (Agreed upon) They could also make supplications for the diseased during the times of answer.
Almighty Allah knows best.