I came up with a less than ideal solution on my own–though I’m going to keep the question open to see if anyone comes with a better solution.
Basically, since I cannot have a brush larger than the 65536.0 unreal units, I instead work with a relatively small landscape, and then scale it to the proper scale once I’m happy with how it looks. This yields a pretty low resolution landscape, though I’m not that concerned as I will place rocks and foliage, etc. which will cover up ugly stretched topology so it shouldn’t be too much of a problem, but I’d be happy to see a better solution for something like this.