It isn't that the melamine isn't porous(or even the melamine at all),it's that acrylic and silicon sealants simply don't work well together and can't stay bonded
Acrylic(what plexiglass actually is)just doesn't bond to silicon(aquarium sealant)so it never forms a completely permanent seal.
The best you could do with aquarium sealant is keep liquids from coming out,but it wouldn't permanently keep acrylic in place.You'd still want to attach the acrylic itself to the melamine otherwise you could end up with things falling apart eventually!