Martin, some of this might apply to you to, if those plugins use the WP_Image_Editor stuff built into WP...
I think the images that are automatically posted to your site should be optimized automatically, although someone recently had a plugin that was similar that was having issues with that.
However, the MetaSlider stuff is going to be a different animal altogether. One thing I had looked at when they released WP 3.5 was the new WP_Image_Editor class. But I never went down that road, because I just didn't see the need for it at the time.
If I implement the WP_Image_Editor class, this might be able to handle all sorts of edge cases (like MetaSlider). Instead of just hooking into the Media Library, it essentially allows me to replace/enhance the built in WP image functions. Soooo, anything that uses those, which most plugins probably do, will automatically be getting their images optimized.
Unfortunately, that's a rather large undertaking, and it will take a while (this isn't my day job after all). But I definitely want to put it on the roadmap for the future.