I suppose my idea would be for the optional variable, if a user is implementing it one would assume they know what they are doing and making sure its properly hosted and accessible, etc. Otherwise it will just default to using your built in logo SVG file.
It would be a great way to offer some slight customization for users who want it, without causing too much more work for yourself – they would be the ones ultimately responsible for making sure it’s hosted somewhere that can be accessed.
Ultimately not a big deal though to not have this, it’s just a wishlist feature.