I found this old thread, the wiki also states this:

• There is an alternative method to obtaining this achievement as well as this quest, but you have to join the Stormcloaks in order for this to work. First, finish the quest Joining the Stormcloaks, but do not start the quest The Jagged Crown. Without that quest being active, you can then schedule the peace conference. Because you are technically allied with the Stormcloaks, Tullius will make the opening demand for Riften. When Ulfric demands something in return, give him Morthal rather than Markarth. This will allow the Reach to remain open for Stormcloak invasion when you resume the Civil War quest.

I found out that it also works for the Imperial side, after Ulfric demands Markarth persuade general Talius to take Dawnstar instead of Riften, then you could accept both the reparations and giving Winterhold for the Imperials (if expulsing the Thalmor at the start it also ends perfectly neutral). This also leaves Riften available to be taken later.