Is there anything I could do to make coda work on sparc64 (32-bit mode
  would be acceptable for now...)?

Someday I will get around to trying to run a coda NetBSD/sparc
userland (venus, probably not codasrv) on NetBSD/sparc64.
[NetBSD/sparc is for 32 bit sparc systems, eg. SparcStation
1/2/5/10/20/IPX/IPC/Classic, and NetBSD/sparc64 for UltraSparcs eg
U1/2/5 etc.]

Coda (venus at least) does work on NetBSD/sparc.
But if Debian on sparc64 cannot run Debian sparc binaries, that won't
work.

-- 
        Greg Troxel <gdt_at_ir.bbn.com>