Hi and welcome!
I have tried searching on the forum and on the net about that "no diag version entry" error, but to no avail. It might be that the battery is broken and the Zaurus fails to boot because it fails to check its status. You can verify this by removing the battery and, with the battery lock open, to power on the Zaurus with the D+M or D+B keys. At least on my SL-C1000, it is possible to enter the Maintenance menu even with the battery not inserted.
EDIT: ant on #kexecboot has suggested that the NAND has been most probably overwritten/deleted, so the only way to recover it is via "Fn + D + M" (which is stored in a PROM) and flash the recovery NAND from Trisoft's page, as suggested by greguu. While in the maintenance menu, I'd run a bad blocks check on the NAND too to check if it has any bad block.
Varti