If I were to use a playpen to put a flat solid mattress down, would this be okay? Any risks I should be aware of?

What age baby/child are you referring to?
Are you part of Child Care Services or for your own child?

A mattress that is recommended for Safe Sleeping is one that is firm, clean, well-fitting & flat.
It is important that there are no gaps/spaces between mattress & sides of cot sides or wall, where a baby might become entrapped.