Currently, the only way I’ve found to do this is using plugins, but all the ones I’ve encountered don’t provide any run-time functionality or aren’t customizable to the level I’m looking for.
How, without plugins, can I insert the avatar of a player (who isn’t in the game) into the game?
If you don’t want to rely on plugins, you can accomplish this by running some code in the command bar. You’ll need to enable HTTPService and use a proxy like rprxy.xyz.
That being said, any and all methods related to fetching the appearance of an avatar only fetches the assets they’re wearing, does not include the rig construct. If you want that, you’re going to have to find the MrGrey models off the Roblox account in the library.