I agree with the OP, sometimes it can be useful to provide some context around the failure to assist the dev, but it doesn't make sense to provide it in the because argument due to where it's integrated in the failure message. As far as I can see, the options available to provide this context when using FA is either to catch the assertion exception and write the context to stdout :

try
{
    result.IsSuccessful.Should().BeTrue();
}
catch
{
    TestContext.Out.WriteLine($"Custom failure message with {result.Context}");
    throw;
}

or use FailWith :

if (!result.IsSuccessful)
{
    Execute.Assertion.FailWith($"Custom failure message with {result.Context}");
}

With try... catch you get both the FA message and the context, with FailWith you just get the context message. I went with FailWith since I wasn't using the because parameter so I could do without the default FA message.

Neither are particularly neat when used to working with the fluent experience ;-) I feel that this could be a lot neater if it was handled in the API e.g. could you add an additional optional param - failedBecause : result.IsSuccesful.Should().BeTrue(failedBecause: $"Custom failure message with {result.Context}")