Thanks for pointing me in that direction, I've recovered the device without needing to attach a serial console.
root@OpenWrt:~# ubinfo -d 0 -n 0
Volume ID: 0 (on ubi0)
Type: static
Alignment: 1
Size: 9 LEBs (1142784 bytes, 1.0 MiB)
Data bytes: 1019796 bytes (995.8 KiB)
State: OK
Name: fip
Character device major/minor: 250:1
root@OpenWrt:~# ubinfo -d 0 | grep physical
Count of bad physical eraseblocks: 0
Count of reserved physical eraseblocks: 20
root@OpenWrt:~# ls -l openwrt-mediatek-mt7622-linksys_e8450-ubi-bl31-uboot.fip
-rw-r--r-- 1 root root 1012564 Apr 25 19:08 openwrt-mediatek-mt7622-linksys_e8450-ubi-bl31-uboot.fip
root@OpenWrt:~# md5sum openwrt-mediatek-mt7622-linksys_e8450-ubi-bl31-uboot.fip
74438e269be0c03049b40fdead43d6dc openwrt-mediatek-mt7622-linksys_e8450-ubi-bl31-uboot.fip
root@OpenWrt:~# ubiupdatevol /dev/ubi0_0 openwrt-mediatek-mt7622-linksys_e8450-ubi-bl31-uboot.fip
root@OpenWrt:~# ubinfo -d 0 | grep physical
Count of bad physical eraseblocks: 0
Count of reserved physical eraseblocks: 20
root@OpenWrt:~# ubinfo -d 0 -n 0
Volume ID: 0 (on ubi0)
Type: static
Alignment: 1
Size: 9 LEBs (1142784 bytes, 1.0 MiB)
Data bytes: 1012564 bytes (988.8 KiB)
State: OK
Name: fip
Character device major/minor: 250:1
Rebooted fine, without being held captive in a freezer