Well, as others have said, unassisted is unassisted, if someone is there to help it is not unassisted. As a doula, you might be held responsible for any complications, if the powers that be have their panties in a bundle about homebirth & midwives.
But, a friend, who just happens to be a doula, I think, it would be a lovely opportunity for you.
Be very sure, it is clear in their minds, that you are there as a friend, with no other expectations of getting them out of a jam if complications do happen. Not that you can't help, just that they don't expect you to have all the skills necessary to be able to solve any problems. And *do not take "compensation of any kind" for your part in the birth, no matter what happens! If you recieve any kind of compensation, you could be deemed to be acting "professionally" which in this case would be practicing medicine (or midwifery) w/o a license!
From the other point of view - I had some uc births, and some w fellow midwives. But, I acted as my own midwife, doing most of my own prenatal care, conferring w a collegue only when I had a question I couldn't figure out on my own. I did not "hire" anyone to attend my births, but did invite my friends to my births. Friends w varying degrees of homebirth experience, from none at all, to nurses, & doctors who happened to be my friends! I guess we just didn't worry about liability so much back then. I just thought seeing a totaly natural homebirth might be a good experience for them, but I was in no way holding them responsible for my care or anything that happened at my births. On the other hand, a few yrs ago I as invited to a birth of a friend, I expected to just come as a friend, so I was surprised how much they seemed to be relying on my "midwifery expertise"! I would have been glad to be her probono midwife, if she had asked ahead of time, so it just threw me off guard, because my own attitude in labor had been so different. I was glad to help a friend.