We found a solution.

We can hook into the process this way.

public class CustomWebProcessor : IImageWebProcessor
{
        private const string QueryStringName = "custom";

        public IEnumerable<string> Commands => new[] { QueryStringName };

        public FormattedImage Process(FormattedImage image, ILogger logger, IDictionary<string, string> commands, CommandParser parser, CultureInfo culture)
        {
            var QueryStringValue = parser.ParseValue<int>(commands.GetValueOrDefault(QueryStringName), culture);
            image.Image.Mutate(ctx => ctx.Brightness(QueryStringValue));
            return image;
        }
 }

and then make a static helper to add it to the builder.

public static IUmbracoBuilder AddImageWebProcessor(this IUmbracoBuilder builder)
{
    builder.Services.TryAddEnumerable(ServiceDescriptor.Singleton<IImageWebProcessor, CustomWebProcessor>());
    return builder;
}