This was possible in earlier beta versions of Big Sur by including an icon file with the name postfix 1016 but has since been removed. I've tried the postfix 11 but still no go. I've filed a feedback report (FB8768688) and suggest you do the same.

I'm also wanting to do this and the previous method seems to have been removed. I have filed a Feedback suggestion report FB8768688 and suggest everyone else does too.