Diablo was a Windows, Mac, PSX game. So unfortunately, no dosbox.

How about instead getting the Summoning Wars game ported to the N900.

http://www.sumwars.org/

It's using Ogre for it's engine and looks pretty sweet at 0.5. Though I tried it under Arch Linux this morning and it was all in German or something.

It's a diablo-esque game. (which really is just a more basic version of Nethack with shinier graphics).

Nethack rules, and there is already a stone soup port as well for the N900, though they aren't real-time like Diablo.

Other possibilities would be Soulfu and SCOURGE.