Not in the Unicode guidelines as they currently stand ':).

They are very explicit that this is not permitted [1].

Also, RE :poop: 1. Huffman encoding philosophy - encode frequently used symbols using fewer bytes. 2. Standards are helpful, :poop: is very English-centric, whereas U+1F4A9 is much more interpretable.

[1] https://unicode.org/emoji/proposals.html#Selection_Factors_I...