Ah, gotcha. That would make for a nice feature, I've added a task for it:
EsotericSoftware/spine-runtimes2468
Until then, if you're savvy enough to change the player code, this is where the centering happens:
renderer.camera.position.x = viewport.x + viewport.width / 2;
renderer.camera.position.y = viewport.y + viewport.height / 2;