Hmmm just looked at ours and the cover does actually come off so it is washable so you could was if necessary I suppose - but I don't see why that would be ok as a cover but not a mattress protector? Although I don't suppose they are very breathable...?
To be fair, if you know anyone with any sewing skills replacement covers would be really easy to run up, it's a bit like a pillow case in design.
Ours like yours won't be using it for naps, so will always be fully supervised...