I wouldn't say it wouldn't work for sure. In that large a tank, it is possible the angel could coexist long term with the seahorses. Especially in a tank that is large enough that you could feed different ends of the tank and avoid food competition. It is certainly a risk, but I don't think it is a completely unreasonable idea.
I would just be prepared to take out all the rock if you need to catch and remove the angel. Either that, or be prepared to move your seahorses to another tank if necessary.
I have a juvenile rock beauty angel in a tank with seahorses. Other than being an aggressive eater, she does not interact with the seahorses at all. I do not plan this to be a long term arrangement, the angel is going into my 210, which is waiting on a part. This species has a dismal record of survival in captivity, so I put it into the most sponge- and macro-filled tank I have, which also happens to house seahorses. After more than a month, the angel does not seem to have adversely affected the seahorses at all.