Latex shields are generally made of foam and covered in latex. So long as they don't have any sort of hard plastic or other material around the edges and fit the size requirements they should pass. No need to add foam over foam.
I'd say find the specs for the shield you're looking at and pass them along to the HoR or whoever approves latex weapons in the chapter you want to use it for. They'll be able to give you a "order it and we'll see" or a "No."
Honestly I have see quite a few of these. They are all generally softer than latex weapons and well within size reqs. If you brought it to me I would just check it was in one peice. I suspect they are officially as provisional as latex weapons themselves.
As a weapon marshal in two chapters I would agree with both Phedre and Simon. If they're built as I've seen described in several places including this thread they stand a good chance of being passed, but sending the specs to the chapter in question is a good idea to make sure you're not wasting your money.