Thanks to Richard for pointing me in the right direction here.

I ended up implementing this by creating a custom attribute like so (NB - namespace is unimportant, but the class name must be this specifically):

[AttributeUsage(AttributeTargets.Method)]
public sealed class MessageTemplateFormatMethodAttribute: Attribute
{
    public string FormatParameterName { get; }

    public MessageTemplateFormatMethodAttribute(string formatParameterName)
    {
        FormatParameterName = formatParameterName;
    }
}

This can then be used on a method, passing in the nameof the string parameter:

public static class ILoggerExtensions
{
    [MessageTemplateFormatMethodAttribute(nameof(message))]
    public static void LogCustom(this ILogger logger, string? message, params object?[] args)
    {
        logger.LogDebug(message, args);
    }
}

This gives us support in the IDE: