Surprisingly, it's actually quite a common occurence around here. Through a collaboration project with another research lab, it was recommended to me that I get a LOR from a scientist at the other lab. When I went to discuss it with her, she told me to write myself one and let her sign it. Her reasoning was that since she already supported my decision to go to med school, it didn't matter in what way she could help me to get there, provided that not too much of her time be wasted. Also, I never spent more than an hour in person with her, so she didn't know me too well. Anyway, in the end, I decided not to do it, because I could never write things about myself the way somebody else can. The point of all this is that it's not unusual practice to write your own letter, but just don't broadcast the fact because it may be viewed as unethical to some. The idea of getting a third person to write or review the letter is a good one.