You are right, for some reason it doesn’t user the same picker control as the other forms.
You can work around it using this code:
// Set value
jQuery("#dffs_ParticipantsPicker").find(".ms-inputuserfield").html("USER DISPLAY NAME");
// Click the resolve picker icon
jQuery("#dffs_ParticipantsPicker").find("a[id$='UserField_checkNames']").trigger("click");
Change “ParticipantsPicker” for your field internal name.
Alexander