Edit: I never posted back, but the sysupgrade method does work for a 493G using LEDE 17.01.4.
Steps taken:
Boot router via TFTP using lede-17.01.4-ar71xx-mikrotik-vmlinux-initramfs-lzma.elf as image.
This is required as the OpenWRT sysupgrade is not capable of operating on this device, so we need a modern LEDE one on hand...
I personally used dnsmasq on a local PC and plugged the router WAN to the PC Ethernet port to boot, plus used a null-modem serial cable to tell the 493G bootloader to boot from network.
pc$ ip address add 192.168.1.10/24 dev enp2s0
pc$ dnsmasq -i enp2s0 --dhcp-range=192.168.1.100,192.168.1.200 \
--dhcp-boot="lede-17.01.4-ar71xx-mikrotik-vmlinux-initramfs-lzma.elf" \
--enable-tftp --tftp-root="/tmp/" -d -u $USER -p0 -K --log-dhcp --bootp-dynamic
pc$ screen /dev/ttyUSB0 115200
(then pick "o" for boot option, and "1" for from ethernet)
This should get you a local BusyBox shell with LEDE 17.01.4 booted in memory. You can kill dnsmasq now.
(set a password as this allows remote login later)
router$ passwd
Done!
Flash with sysupgrade using lede-17.01.4-ar71xx-mikrotik-nand-large-squashfs-sysupgrade.bin as image.
Plug PC into the LAN port on the router now, and get an IP from the newly booted router via DHCP.
Also of note, unlike the OpenWRT install (which simply un-tars a filesystem snapshot into mtd6 "rootfs" as jffs2 on /) this sysupgrade gives one the standard flash layout (with a read-only squashfs on /rom and an overlay ubifs on /overlay).
This is a much more satisfactory installation IMO.