U+E000–U+F8FF, U+F0000–U+FFFFD, and U+100000–U+10FFFD can already provide you with your own emoji, as that range has been reserved for private use. Extending the range further might make sense if you need even more space in your program, but that's a lot of space already.

2-3 bytes are not much space for anything. Sure, you could use multiple successive ones of these code points and define your own "continuation" encoding in these ranges, but that doesn't seem right to me somehow

The point is probably to encode the emoji (image) in the codepoint itself.