I am aware of the following in the App Review Guidelines;
3.1.1 If you want to unlock features or functionality within your app, (by way of example: subscriptions, in-game currencies, game levels, access to premium content, or unlocking a full version), you must use in-app purchase. Apps may not use their own mechanisms to unlock content or functionality, such as license keys, augmented reality markers, QR codes, cryptocurrencies and cryptocurrency wallets, etc.
The app itself operates normally, but imposes limits after a limited time of use. Purchasing the full version removes these limits. This is a one-time IAP, not a subscription.
My app is aimed at professionals within the entertainment industry, and will only ever be used within a theatre or other similar venue.
I do not intend to provide any other paid route to the full version than via the App Store.
However, what I would like to do is provide free use at certain events and to students in their educational venues (e.g. college theatres etc)- is it acceptable to use an external mechanism (e.g. a server based API) to temporarily remove the limitations whilst in that venue? Or, likewise, provide an extension to the free trial period for specific uses.
Note this is not a purchase, as there is no payment. Therefore there is no revenue outside of the App Store (which surely is the point of the mandate to use IAP for this purpose)
Also, it's not fully 'unlocking' the app (the user doesn't get the full version, they just aren't bound by the trial limit timer when within that venue or for a limited time)
I'm aware that another route would be to put a different version of the app in the store that requires a username and password for use (for example) and then provide those credentials to log in - as there's lots of apps that follow that model, but it seems clunky to require the user to download a different app.