the official nomenclature for this sorts of borrowing
By whose authority?
ie like 自 for 鼻
These never loan each other in pre-Qín texts. People speculate that 自 once also wrote a word that meant "nose" in addition to "from" and the reflexive verb marker since it looks like a nose in older forms of the script, but that usage is completely unattested.
No good sources say that 足 borrowed to write "enough"
What sources are you even looking at? It seems like you don't understand much about pre-Qín writing. 足 writing two homophonous words that mean "foot" and "enough" is common knowledge. Trying to etymologize some single word origin for "foot" and "enough" because they are written with the same Chinese character is putting the cart before the horse.