They should do. If they don't they're not spec compliant. I'm not sure what they actually do though. Currently only Bitcoin Core and Android Bitcoin Wallet implement BIP 70 so you can just create such a request and then try it out and see what happens.

On Sat, Mar 29, 2014 at 4:02 PM, Justus Ranvier <justusranvier@gmail.com> wrote:

On 03/29/2014 01:30 PM, Mike Hearn wrote:
> They would just encode the OP_RETURN script into an Output structure. I'm
> not sure about the question - you seem to give the answer yourself in the
> first paragraph?
>

I guess what I was asking is whether or not all BIP70 compatible clients
will support the creation of all standard output types, including
OP_RETURN outputs.

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