The frozen waffles worked great & I think fresh waffles would work too – they would just need to be a tad bit crispy. Soft waffles might not hold up as well but with baking the eggs, there’s not really any liquid coming from the eggs to make the waffles soft.
We made the sandwiches too, using english muffins for us & the waffles were primarily for my daughter, even though I think my husband might have eaten more of them!! 😉