Well, you named one example of a belt being tensioned outward. Ya got me!
I'm sure there are others. I worked on a lot of used cars in the 60's and
recall seeing what you described, but never on anything else.
It also looks a bit dangerous. If you need to fuss with the idler-pulley
it is much more dangerous to stick your fingers inside the belting than
outside. Just imagine being startled while your fingers are inside the
loop diddling the idler pulley. With a small steamer, ouch. Anything bigger...
For intuitiveness, ease of construction, and just to look right....
on a small setup, like a go-cart or minibike, I go with the idler outside
the belt. Now, I have to make a clutched Baker fan. I already have
a idea for my idler-pulley. Maybe my Erector set can help for both.
Be careful,
Wayne