I hate to limit non-game engine usage of Spine, but we try to keep features as widely supported as possible. Bezier for clipping could work in the editor and export to images/video, but people would want to use it in runtimes. This creates confusion for users and support load for us.
You can export to images or video and bring that into After Effects, as it sounds like you are already doing. Many users do this, especially for cartooning and similar usage.
We've considered an After Effects runtime, but unfortunately we don't have that in our short term plans.
There are some workarounds, depending on your exact needs. It sounds like you've explored some of these, but found them time consuming. There may be other ways to structure your project that are less time consuming, but it depends on your specific needs.