The following can not answer the question "was there ever a case", but only describe what is considered best practice. I just learned that humanitarian negotiation is actually a well-defined term in the context of international organisations:
Humanitarian negotiation can be defined as the interaction between humanitarian organisations and their counterparts to:
- establish and maintain the presence of humanitarian organisations in crisis environments (conflicts, disasters, migration flows, epidemics…)
- ensure humanitarian access to people in need, and
- deliver humanitarian aid and implement protection activities.
Humanitarian negotiations have a relational component, focused on building an ongoing relationship of trust with counterparts, and a transactional component, focused on establishing and agreeing on the specific terms and logistics of humanitarian operations.
Humanitarian negotiations are different from peace-making or political negotiations, which are usually carried out by mediators or diplomats. Their purpose is not to influence political or diplomatic positions but to ensure humanitarian agencies have access to people in need to provide humanitarian assistance and protection.
This is an excerpt from the profile of the Centre of Competence on Humanitarian Negotiation, a "global network of frontline humanitarian negotiators" and "a joint initiative of five humanitarian agencies: the Centre for Humanitarian Dialogue, the International Committee of the Red Cross (ICRC), Médecins Sans Frontières Switzerland (MSF), the United Nations High Commissioner for Refugees (UNHCR), and the United Nations World Food Programme (WFP)."
Field representatives of these organisations have the role of "negotiators" for humanitarian principles vis-à-vis the agressive "counterpart(s)" (could be one or two warring parties), they should have a "support team" and are normally acting on behalf of the "mandator", the humanitarian organization. According to their comprehensive Field Manual, "a set of concrete tools and methods...to plan and prepare negotiation processes":
Humanitarian negotiations are team-based, comprising the frontline negotiator who leads the engagement with the counterpart, the negotiation support team who assist in the critical reflection on the orientation of the process, and the mandator who frames the process into the institutional policies and values.
The presence of observers or witnesses is not taken into systematic consideration. Instead, the avoidance of misunderstandings and malicious interpretations is abstractly discussed as "determining the Common Shared Space".
So yes, there are regularly non-warring parties involved in negotiations about humanitarian concerns between warring parties, but their usual role is nothing like that of a silent witness, or a neutral mediator. They are an interested party, and normally they are even the ones demanding negotiations out of humanitarian concerns.
Because it is so eerily famliar, I'll quote one example the manual uses:
Siege Negotiation: Tripartite Negotiation with the Besieging Party and the Besieged Opposition
In Country A, most of the countryside is under the control of an armed
opposition group. To gain access to the population under the armed
opposition’s control, FWB [Food Without Borders, a hypothetical organisation] must negotiate concurrently with the
government of Country A and the leadership of the armed opposition as
the convoys move regularly from government-controlled to
non-government-controlled territory.
In this case, the government’s main interest is political, i.e., to
avoid providing further legitimacy to the armed opposition through the
access and distribution of food by FWB in the territory under its
control. Additionally, the government wants to collect data on the
population being served and obtain lists of beneficiaries.
The leadership of the armed opposition is also eager to gain
politically from the distribution of FWB food as this assistance will
contribute to ensuring a greater cohesion of its political and
security alliances with tribal leaders in the various communities. The
opposition leadership wants to control where the distribution takes
place and is opposed to the transmission of population data to the
government as it suspects that these will be used for intelligence
purposes.
For its part, FWB is eager to maintain its access and proximity to the
population. FWB wishes to maintain control over the distribution of
food to the population most in need. Since there have been concerns
about diversion, it wants to monitor the distribution site. It is
aware that the lists are becoming a political issue for both the
government and armed group.
As the point of the example was only to point out that there can be multiple counterparts in a negotiation, there is no further explanation on how to proceed beside a short "you will need all the tools that you know"...
