www.firespeaking.com has a whole page devoted to mass heaters combined with staircases, which seems like a great idea to me. Both mass heaters and staircases take up quite a bit of floor space, so if you combine them into a unified structure they conserve space and add a lot of interest to the room. Below are some of the examples that are on that page.

Staircase Heater by Pál Szentgróti of Szentgróti Manufaktúra

Heater by Flemming Abrahammson of Fornyet Energi

Staircase Heater by Warmstone Heat. Built by Ron Pihl, Dale Hisler and Jerry Jessen.

Back of Staircase Heater by Firespeaking

Heater by Flemming Abrahammson of Fornyet Energi

Heater by Flemming Abrahammson of Fornyet Energi

Staircase Heater by Roland Voppel of Vuurmeesters (photo credit: Fred Roest)