Can you seize a company's intellectual property if it was created using your platform? Can you put something in the user agreement that allows you to seize any intellectual property created using your platform? Let's say you have a game called Warcraft, and the Real Time Strategy game allows people to create other games out of it. Can you forbid companies or people to take their games out of your game or platform and make their own game using their own platform or someone's else platform, or take ownership of the intellectual property? I am thinking such an abusive user agreement would not be upheld in a court of law, but I could be wrong. Let's assume this is in the United States?
First, you cannot seize anything. The courts can, and you can ask the courts to do so for you. So the logic of this is that you operate a website that hosts software that creates something for a user. You could just unconditionally let people use it, you could require a subscription, or you could be like SE and put up a TOS with some conditions, like "you grant us a perpetual license to use your stuff". Users then have to agree to this in order to get access to your software.
So the answer to your question depends on whether you can write a legally-enforceable contract where you provide something of value in exchange for a user giving you something of value. Then if you hold up your end of the deal and the customer does not, you sue them in court for damages. The main questions are, is such a contract enforceable, and what would be the remedy that the court might order?
Your description of your scheme is a bit sketchy, so I'll fill in some details. The user must submit a functioning software package, which your service plugs into some slot on a platform and makes accessible to others. That's it. In exchange, the customer transfers copyright to your company. This is unenforceable (in the US), because a copyright transfer is governed by 17 USC 204, which required a written instrument of conveyance signed by the rights owner. This precludes Human Centipad trick-transfers – the rights holder has to actually know and agree.
Next pass, your TOS requires the rights holder to grant a perpetual, unpaid exclusive license to use the stuff. That means you still hold copyright, but you can't do much with it. There's a revocation window open for 5 years, 35 years later, a bit late in the software game. Well, the courts treat this as a copyright transfer. The similarity between your (modified) idea and Fathers & Daughters is that the original copyright owner is not a "beneficial owner" because they do not receive royalties. Again, I take it your goal is to sneak in the effect of a copyright transfer without alerting the user.
Finally, you include a royalties clause: we've got a contract. But now we've strayed from the notion of "seizing". Perpetual licenses are legal, exclusive licenses are legal. You would have to sue the copyright owner for damages if you grant a perpetual exclusive license (for money) and then they grant someone else a license to use. You as platform operator be awarded whatever you had arguable lost from their breach of contract. The courts would not "seize" anything other than, perhaps, the other party's assets.