Ines Zemelman, Founder and President, Taxes For Expats, replied:
From a U.S. tax perspective, if you work as a freelancer (without incorporating a business), provided that you pay into the Belgian Social Security system, you will be able to exclude your self-employment earnings in the U.S. as Belgium and the U.S. have signed a totalization agreement. Prior to the tax reform of 2017, there were US tax benefits to incorporating your business outside of the U.S; these mostly went away with the forced repatriation of foreign untaxed earnings.
If you obtain a Eenmanszaak or Entreprise individuelle – this will require the filing of Schedule C. If you have a SPRL or EBVBA or BVBA – this would require Form 5471 (a foreign corporation by the IRS classification). If you have a VOF or SEP or SCS, this would entail filing of Form 8865 (foreign partnership, more complex filing than under Entreprise individuelle – same US tax consequences as described above).