And in such a case, an Open Source mandate would directly conflict with the financial interests of the investors (the time required for the effort takes away from the liquidation amount of the remaining assets.

They'd have to be left out. The rest could be open-sourced. If 3rd party stuff is hidden behind API, it could even be replaced with open-source stuff. That's not even necessary as my recommendation is to not overly burden the startups with this requirement. They just dump what's theirs somewhere under FOSS license. Others can do the necessary work of dealing with missing, 3rd-party components if they want to reuse the FOSS code.