Not on the microSD, no. All the uboot menu items go in /etc/bootmenu.d/ on the phone itself. Assuming you have extracted Arch to the ext2 parition, (I don't know if u-boot reads FAT?), I believe you just need to change: root=/dev/mmcblk1p2 to root=/dev/mmcblk0p2 as the block numbers are not the same as in maemo. Give that a try, just don't forget to run u-boot-update-bootmenu afterwards.
$ u-boot-update-bootmenu ~sh: u-boot-update-bootmenu: not found