Sleeper reservations with rail pass?

We wish to travel from Osaka to Hokkaido on the Twilight Express and from Hokkaido to Tokyo on the Cassiopeia during October. It is not possible to reserve these seats in advance from the UK with a railpass and a Japanese friend has not been able to find domestic Japanese reservations for us.

What is the chance that we will be able to reserve when we arrive in Japan? Is there a special reservation of places for rail passholders?

There are no special seats reserved on trains for JR Rail Pass holders only; and otherwise it is first come, first served. To make reservations, you'll need to show your rail pass and that is maybe why your Japanese friend didn't succeed in making reservations.

If you plan on booking a couchette or private room, have a look at this page: http://www.japan-guide.com/e/e2356.html

Any JR Pass holder has to pay a surcharge for sleeper berth use (Sleeper berth charge and limited express charge, approx. 9,000 to 11,000 yen). So your friends story sounds a little weird. He or she can purchase the sleeper berth ticket with a limited express charge without showing any JR Pass in advance for you.

You might ask your pal to purchase the tickets for you again.