I would guess that jsonb columns or Entity-Attribute-Value type of schemas are permitted, but if doing something in a grey area it wouldn't hurt to reach out to

I would guess that jsonb columns or Entity-Attribute-Value type of schemas are permitted, but if doing something in a grey area it wouldn't hurt to reach out to the company and get permission in writing (or negotiate a license exception.) Or just pay for their managed service, you probably don't want to be fiddling around with hosting the database yourself. At least everywhere I've worked we used RDS for precisely that reason.