I have no facebook but maybe you do? Could be you can find the companies there and could try to get a response to your question.
Maybe try to find an active community / forum of one their games and ask if someone there knows an active dev or company representative.
For your consideration:
If you make your efforts public there may be no way back because they may start to follow your creation.
When you keep it small and keep treating it as nothing more than a community mod played by only a handful of people they may never know or care or take any action. In the end HD2 is a very old game, there are obviously no plans to make a third installment and even if they did everything you use of the game is so outdated it would be hard to believe you are getting in their way. All they will ever care about is the name of the game, Hidden and Dangerous. Just my 2 cents.
If I can give you any advice on your project my advice would be this: Follow Crowbar Collective path
They started with this big idea to remaster Half-Life and look what it is today.
They are not official Valve Software employees and they had NO rights for Half-Life series
they came up with Black Mesa name after the "Black Mesa Research Facility" in Half-Life.
If you won't gain a permission to use "Hidden & Dangerous" or "Hidden & Dangerous 2" etc etc. You have to come up with some name.Hidden in the Shadows? OrShadows of Deathdamn idk without that permission it could be hard to make up new name there
NL - Maybe try to find an active community / forum of one their games and ask if someone there knows an active dev or company representative.
2K Czech (Illusion Softworks) have a facebook group try to contact them there madtec