It's definitely possible with php, I swear I saw a very similar project on vB 3.x somewhere before (I'll post it here if I find it).

However, as you mentioned, it would probably require way more bandwidth to support such feature.
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I think I found it: https://www.vbulletin.org/forum/showthread.php?t=201228

It might not be compatible with newer browsers but it's definitely a proof of possibility.