Question on MPO and Partner Quotes, what is your best practice?

Hello community, a question regarding how does MPO accelerate your deals? 

Today, direct to customer private offers can accelerate deals by using the private offer process for a customer to "electronically sign" a document attached to the private offer. 

However, for MPO we do not have a way for a channel partner (aka: reseller) to "electronically sign" a partner quote via Marketplace because MPO does not have a way to attach a pdf that the partner exclusively sees & accepts. In other words, any pdf we attach is also visible to the end customer which we do not want. As a result, we struggle to use MPO to accelerate deals (note: we are in discussion with partners about using REO but they also want to use MPO)

Have you considered the MPO partner quote scenario and if so, have you found a way through this to accelerate deal making? What do you do?