www.skybus.co.jpThis looks promising but IMPOSSIBLE to work with here on the ship, a ton of graphics on their website. The operate in the Sapporo area and do transfers and day tours.
Pricing much higher of course than trains and buses but might be doable and so much easier.
A day tour from Sapporo to Otaru to Sapporo for 8 hours for 5 people looks to be about 45000 yen and I believe that is about $500 or $125 a person for four people. That should work fine. Do a little touring on the way to the ship and just get dropped off in Otaru. I could see this pricing on the web. No pricing for two people.
I did send an email asking for a transfer for two or four people from Sapporo to Otaru as I could not get their transfer rate quote page to load.
If anyone wants to check the site and post here that would be good and when I get home I will look further but this does look very promising.