optional/null-able OUT parameter in C#

That looks fine to me. A out cannot be optional for technical reasons (it needs to point to a valid instance).

Since C# 7.0 you can pass a Discard (_) into the method:

return IsPossible(param1, param2, _);

See: https://docs.microsoft.com/en-us/dotnet/csharp/discards

A 'ref' or 'out' parameter cannot have a default value.

The way you've done it is just fine. Sorry, no silver bullet.