Re: filter at the telco level?
OK, assuming some sort of signature based pattern can identify the infected video, why involve the telco at all? That would mean that the hangouts app itself could perform the scan before sending it off for preview. This is important, because hangouts can be pushed through Google play as an update.
Although it wouldn't eliminate the attack vector (too much insufficient storage-esq errors on old devices), the attack surface would easily and quickly halve.
OK Google, you've got 90 days.