How good or bad would chainsaw bar lubricant be as a bicycle chain lube?
On a chainsaw is it called bar lube (not chain lube).
It is designed to lube the bar.  

This is chainsaw chain:

Not the same beast as bicycle chain. 
I don't even think there are rollers. 
Bar lube is more viscous than bicycle chain lube. 
Bar lube does not need a long life - it is going to get thrown off. 
There is an excess of bar lube and big gaps compared to a bicycle chain. 
The chain itself only needs to last longer than the cutting teeth.

Yes they are both chains but that is about all they have in common. 
It is a bar lube.

I would chose 3 in 1, motor oil, or transmission fluid over bar lube.
Yes I would use bar lube on my bicycle chain if I had nothing else in the shop but I would not use it just because it was free. I would even use WD 40 for a single ride and get chain lube the next day. Not a great lubricant but WD 40 would not gunk up the chain.

You pay $20+ for a decent chain. Why would you skimp using a product not designed to protect the chain? A good chain lube is not very expensive and goes a long way.