The backspace symbol makes perfect sense. You may very well want to say "Press ⌫ to delete the selected object".
To actually get the character included, you can't just reason that it might be used in a certain way, you will have to demonstrate it with actual and authentic use in the wild. The accepted proposal for the inclusion of the power symbol is a nice example of how this works.
If you can find a bunch of printed manuals or books or websites that actually use this icon in this manner you might be able to submit a successful proposal.
I mean, you might want to say "External links can be identified by their □ marker" or "The Windows key is marked □ and can be found to the left of the space bar"