Using the following command line causes Emacs to briefly display the initial frame, then hide it:
emacs -Q --eval "(setq initial-frame-alist '((visibility . nil)))"
This is consistent with the manual:
If these settings affect the frame geometry and appearance, you’ll see the frame appear with the wrong ones and then change to the specified ones. If that bothers you, you can specify the same geometry and appearance with X resources; those do take effect before the frame is created. See X Resources in The GNU Emacs Manual.
Which X resource should I use to make the initial frame invisible? Is there a cross-platform way to do so? On GNU/Linux, how can I set these resources for the duration of a single command?