It's kind of possible to do that, but I don't know the specifics. Like you can select chrome provided webview or system provided webview. However, looks like gecko is explicitly not providing API compatibility, so looks like a no go from start.
Having looked it up at some point, as far as I can remember the list of permitted Webview providers is hardcoded to the official webview package and the various branches of Chrome (Release/Dev/Canary and whatnot).
So you can only use an independent Webview if you're also building your own ROM at the same time, and then you still need to reimplement the whole exisiting Webview API, which could be rather fun if parts of it turn out to depend on Blink-peculiarities that cannot be easily replicated in Gecko.
And finally (and this applies to a standalone GeckoView package without direct API compatibility as well), end users being able to update the Webview independently from the apps using that Webview package means that you're suddenly operating under much more severe backwards compatibility constraints.
