Is it permissible for an employee to receive a financial bonus in the form of a work secondment, knowing that this measure is in appreciation of his work efforts and with the approval of his superiors?

If your managers have the authority to reward you by granting you a secondment, then there is no objection to that. However, if it is an act of favoritism from them towards you, or a violation of the work regulations, then it is not permissible.