Yes, a 0% loan between two Belgian PLCs is possible, under certain conditions.
A written document signed by both companies is sufficient
It must include: the identity of both companies, the amount, the date the funds are made available, the term/due date, the interest rate (or "interest-free") and the repayment terms.
No interest rate is legally required
Belgian law does not impose any interest rate: it is an optional element of the loan.
Two risks to watch out for: misuse of company assets and abnormal or gratuitous advantage
Misuse of company assets: the loan must serve the interests of the lending company.
Abnormal or gratuitous advantage (tax): the absence of interest must be justifiable in relation to market conditions.
For a modest amount and a short duration, the risk remains limited if repayment is certain and the transaction is justified (group relationship, cash flow support, etc.).