Isn’t this question better directed toward OCA people, rather than Copts? I don’t know what counts as economia for the OCA, but I do know that the OCA is one of two EO churches (the other being Bulgarian) in my home area that has a significant number of Ethiopians and Eritreans in it, as they’re essentially the entire OO community in my home area, which is far away from an OO church (hence I moved 1,200 miles from home in order to finally go to a Coptic church). I have heard from some EO online that such people are not accepted without conversion (via profession of faith, from what I gather), but that doesn’t quite match the situation back home from what I’ve asked of the Ethiopians I actually know (nor the situation that has been described to me by EOTC acquaintances in other places, though of course you’ll always find variation, so I don’t mean to imply anything about the OCA by stating a few personal experiences).
I would think that the most accurate thing I could say given the way the question is worded is that it is by no means impossible, but where you do find it there’s generally something else going on that explains why a particular situation is as it is. I’ve posted about this a bunch before, but when the Copts first started showing up in Albuquerque they communed at the local Greek Orthodox Church until they got a Coptic priest and were able to hold their own liturgies. This is probably a not uncommon situation in certain parts of the diaspora, given the relative size of any given OO communion outside of their traditional countries when compared to the older and more established Greek, Russian, etc. diasporas in the West. The situation that you mention with the Coptic family who is hundreds of miles away from the nearest Coptic church is also very common. The nearest Coptic church (where I was baptized) is 6 hours away by car, and that’s where our priest comes from to celebrate liturgy with us every few weeks. There really aren’t closer OO churches, thanks to the geography and demographics of this region (I know of the Ethiopian church in Denver, but that’s also about 6 hours away, and the churches in Texas are of a comparable distance thanks to Texas being huge).