It’s not really safe / possible in a cooked build. All of geometry manipulation code is editor-only right now. You could adjust raw ‘cooked’ FVector4 data and recreate render proxies each frame but it wouldn’t be very performant. Instead, I’d recommend either animating scale of component (if you just want sprite to stretch/squash) or making a custom material for anything more complicated. You can use full material pipeline of Unreal to do crazy complicated things on a sprite.
I’d be interested in specific use cases though, it would certainly be possible to add a kind of sprite component that lets you dynamically adjust geometry if there is enough interest, ala CustomMeshComponent for 3D meshes.
Cheers,