01/27/23 Welcome, doctorscoop. If your question is whether you can request the marriage license -- habilitação de casamento -- through a proxy by giving that person a Power of Attorney to do it for you, then participate in the wedding ceremony yourself on your own behalf and sign the final wedding certificate, rather than your proxy signing for you, your fiancé(e) needs to ask the cartório that question and get a definitive answer. If the cartório will allow you to start the process with a proxy and complete it yourself, you may be in the clear. If they insist that the process must be completed as a proxy marriage, though, your being present as a witness probably won't help you: you'll run afoul of the black letter law that forbids granting immigrant status on the basis of a proxy marriage.
There are two ways around this. The first, probably the most straightforward and the one that involves the least plane tickets, is for the two of you to get married in the United States, and register your marriage immediately at the Brazilian Consulate responsible for your state. You will receive a "Certidão Consular de Casamento" from the Consulate, which your spouse on returning to Brazil will register with the cartório, and after 7 to 12 days receive a Brazilian Marriage Certificate in return. (My husband and I did that, and found it really simple. Check your county's rules on marriage of foreigners, though.) You can apply for your VITEM XI at the Consulate as soon as you register the marriage, so that's taken care of. This is also the easiest way to make sure that your marriage is registered in the United States, because US Consulates in Brazil do not register Brazilian marriages. They recognize them, but marriage in the US is a state concern, so the State Department has no jurisdiction, and you'll have to find a way to register your Brazilian marriage in your home state at a later date, if you want it on the record there.
The second way is to take advantage of the delays in the cartório system which most people complain about, but in your case, can work to your advantage. There's ordinarily a delay of 15 to 30 days between the request and the issuance of the habilitação de casamento. Once it's issued, the couple has 90 days to complete the marriage ceremony and finalize the paperwork. So if you're well organized and have your papers in order, you could come to Brazil for a few days to request the habilitação, and then go home and return for a few days up to three or three and a half months later for the actual marriage, and not have to mess with proxies.