FYI, the mentioned extlinux.conf has an “APPEND” key/value pair, which is where you’d add content. You could create a second boot entry instead of modifying the first entry if you want some safety. In that case you’d be able to use serial console to pick an entry at boot time. Although this is not something you’d want to use for remote use you could make your modified entry the default. If there is a problem, then you’d still have the other entry for worst case.
Also note that the “${cbootargs}” is content inherited from earlier in boot. That content is mainly from the device tree node “chosen->bootargs”. Those entries are used by earlier boot stages, and could in fact have edits before being passed to later stages, so you would not necessarily want to edit the device tree, but there are cases when you would.
In newer releases I think you can add a new device tree as a file by adding an FDT key/value pair entry within extlinux.conf. Without the FDT entry it should pull device tree from a partition, but if you want to test and avoid flashing partitions, then the way to do this would be with the FDT entry. If the original entry is left in place and you use FDT in a different entry, then you have a back up plan if the FDT edits fail.