score:4
I can't prove a negative, but as far as I can tell by looking at the sites of A-Line and Marix, the answer is no, they don't offer any sort of hop-on-hop-off ticket, nor do you get any "stopovers" if you buy a longer ticket.
There's a third option from Amami to Kagoshima, the once-weekly Tokara Ferry via the tiny and obscure Tokara Islands (long on my personal to-do list), but this costs even more at ¥11,620 one-way.