This is tricky territory. Assuming you're staying completely within the Agenda and Principles, you're not permitted to lie to the players about what's happening. You're also not permitted to have secret plans that aren't yet part of the actual play. This limits your ability to have unrevealed secrets that you know are true. The nature and fact of a PC's insanity, as you're describing it, breaks the MC's rules.
However, it's awesome and is totally the kind of thing that should come out of a cheesed-off Psychic Maelstrom, so the question is not whether, but how to do it.
What you need is an insanity where you can:
- Reveal the experience of insanity directly to their senses
- Not predetermine that it actually is an unknown insanity, so the truth of the experiences in (1) can be explored during play
- Be honest at all times
This pretty much eliminates any "what is going on???" experience you want the players to have around an insanity that they don't know their PC has.
So let's see how you can do this, by going through those items backwards.
The solution is to not hide the insanity from the player. Instead, make them complicit with you in creating the behavioural effects of the insanity. To do this, you can't just say they're insane and expect the player to run their PC that way, because doing what you tell them is categorically not the player's job in Apocalypse World.
To get complicity, bribe them mechanically while being brutally honest. Describe the sensory experience, honestly say that it's clearly not real, and then offer them XP if they act as if it is and ask "What do you do?" They don't have to take the situation at face value then, and retain their player-agency. Also, if they're intrigued (and what are you doing, offering situations that don't intrigue the players), then they'll be very tempted by the chance to mark XP and may just dive right in.
And if they don't dive right in, that's fine. They still have to answer "What do you do?" and their actions will snowball like normal. Maybe they try to focus and see what's "really" going on (sounds like acting under fire, yeah?); maybe they flee the scene and go try to get their head together; maybe they try to ignore them and make a guess at what's really happening. Either way, you can build on that, and they retain control of their PC's choices — the bedrock of an Apocalypse World campaign.
Don't assume you know it's insanity
So the Brainer, Iris, has been messing with some weird Maelstrom mojo and you figure this has exposed them to insanity, but you're not straight-up saying "so you're going kinda insane, how do you feel about that", instead you're dropping these weird events and visions on them and seeing how they react.
You might be wrong. They might not be insane. The rules dictate that you cannot decide what these events actually are, because you have to play to find out. Maybe Iris goes to Fanny Malone and flops down into the ambulance's gurney, demanding for Fanny to figure out what's going on with her head. Fanny goes to work like a Savvyhead on Iris and woah, the scene(s) that result reveal that Princey's planted a chip in Iris' head.
So you play to find out what happens, because that will be way cooler that whatever pre-planned stuff you were cooking. Unlike soup, more cooks improve the game instead of ruining it, so you gotta let go your plans and assumptions and follow the game where it leads you, just like you do when you're a player with a PC.
So go with the insanity thing, yeah. Have it in your back pocket for inspiration of things to tell your PCs they see, hear, whatever. But as soon as play pulls you away from that being the answer to what's going on? Put your idea that it is an insanity in the crosshairs and pull that trigger, like a good Master of Ceremonies. Your players will thank you, because damn but does railroading suck the fun out of this game.
Tell them truly what they experience
Get real vivid and barfing-apocalypse-y with your descriptions. It sounds like you've got some weird, evocative imagery up your sleeve — use that. I know I said don't plan anything, but planning visuals is cool, that works, because visuals can always have their truth explained later, and in real play you always adjust, toss, and repurpose planned visuals anyway. You don't get stuck on them, the way you get stuck on secrets and plans, so they're not verboten and dangerous to MCs.
So barf that apocalyptica, and see where your players take it, and make no commitments about what it truly is until you're honestly revealing what it truly is.
Bribes! Really really: Bribes!
I'm repeating this, because I cannot recommend it strongly enough. Players love XP as a bribe. They will do the damnedest things in the heat of a scene with an XP riding on their choice. And if they don't take it, hooboy, have they ever made a strong statement about their character then!
Offering XP for choosing obviously-bad courses of action is pure gold. Try it a couple times if you're not convinced, and you'll see how nail-bitingly charged those moments become, and how big the snowballs can get.
And as a benefit to you, it takes all the responsibility off you: they are entirely in control of their destiny at that moment, working with good information, and any road to hell they are about to set their feet upon, they will never give you grief for because they chose it with eyes wide open. That makes MCing so much less of a weight. Share the load — bribe your players!