Hmm, besides substance you'd definitely need to give it the appearance of legality.
Messi could have SOLD his image rights to the offshore companies in Belize and Uruguay for a reasonable sum, and pay Spanish taxes on it. The contract may state, for example, that Messi gets XXX € in advance plus a yearly allowance.
With the appearance of legality and some substance in Belize and Uruguay, you may have a decent setup, and even Spanish Inquisition may have a very hard time to demonstrate that you are committing tax fraud.
"Mr Judge, I needed some money very fast, and those Belize Thugs screwed me. Now I'm their slave until the end of my life: they get all the money and I get the breadcrumbs". It's their job to prove that the same person is also behind the Belize offshore company, and if you do things properly that's not so simple.
As
@Tax Cow often reminds us, you must pay some taxes somewhere, or be ready to face the consequences...