Sorry about the cross post, but is it possible to mount
   a NetBSD/alpha partition on a NetBSD/sparc system?

one can use `options FFS_IE' in ones kernel to allow mounting the
opposite-endian filesystem types.  i haven't done it alpha ->
sparc, but i have done sparc -> alpha *several* times.

i used to quite successfully for months, but then i had some
strange problems so i backed up the data and use fsck(8) to
convert the endianness of the systems (this is a non-
interrupable process, unless you want a dead file system, hence
the reason for backups.)