Oddly, there is already even a setting for exactly this:

Provide members with posting guidelines
Posting guidelines let people who are new to your community or posting for the first time know what your expectations are. If you have specific flair or formatting requirements for posts, this is the place to make it clear what you’d like.

I'd always thought the text in this box was supposed to be shown on the create-post screen; but it seems that's not the case after all.

Indeed, your concern is something we have already seen; some users do end up posting their NSFW videos to communities that are not properly tagged as NSFW (even worse, these are often ones created by scammers) instead. So this essentially further punishes the NSFW communities that do follow the rules.

Can you also confirm that this will be fixed so our post guidelines will also appear? Most of our posts are videos (or at least, they used to be before this happened), so we obviously need some means of telling people how to actually post them.

What I'm asking is, how can we tell users that? The message doesn't say to upload off-site, it says to go "pick another community". This message isn't customizable, and posting guidelines are not supported on the app at all.

As I said above, media posts are enabled in our settings, but Reddit does not allow media uploads regardless of this setting if a community is NSFW. So Reddit is telling our users to go somewhere else, over something we've got no control over; and users who do go elsewhere would find every other NSFW community displaying the same message.

Our Posting Guidelines briefly explain this and tell users where to host videos to post as a Link Post; but these guidelines are not shown here either, even though they clearly should be!