Is it possible to get the reverse of this working? (Waydroid with play services on Linux phones, such as postmarketOS)
Why wouldn't it? All you need is a binder device for Android IPC and root access to launch Waydroid. It should work perfectly fine when installed and used with Wayland.
I've been using Waydroid with microG on a Librem 5 with PureOS for years. Not extensively as I don't have a lot of reasons to boot Android, but when I do have one it's there.
I've seen some guides for installing Play Services in Waydroid, but personally I'm not interested.