Custom Binding start place in https://github.com/slodge/MvvmCross/tree/vnext/Sample%20-%20SimpleDialogBinding/SimpleDroid

Setup.cs

using System;
using System.Collections.Generic;
using Android.Content;
using Cirrious.MvvmCross.Binding.Binders.Json;
using Cirrious.MvvmCross.Binding.Droid;
using Cirrious.MvvmCross.Binding.Droid.Binders;
using Cirrious.MvvmCross.Binding.Droid.Simple;
using Cirrious.MvvmCross.Binding.Interfaces;
using Cirrious.MvvmCross.Binding.Interfaces.Binders;
using Cirrious.MvvmCross.Binding.Interfaces.Bindings.Target.Construction;
using Cirrious.MvvmCross.ExtensionMethods;
using Cirrious.MvvmCross.Interfaces.Converters;
using Cirrious.MvvmCross.Interfaces.ServiceProvider;

namespace SimpleDroid
{
    public class CustomBindingParser : IMvxBindingDescriptionParser, IMvxServiceConsumer
    {
        public IEnumerable<MvxBindingDescription> Parse(string text)
        {
            // TODO - parse text
            return new List<MvxBindingDescription>();
        }

        public MvxBindingDescription ParseSingle(string text)
        {
            // TODO - parse text
            return new MvxBindingDescription();
        }

        // TODO - maybe refactor this somewhere else?
        public MvxBindingDescription SerializableBindingToBinding(string targetName, MvxSerializableBindingDescription description)
        {
            return new MvxBindingDescription()
            {
                TargetName = targetName,
                SourcePropertyPath = description.Path,
                Converter = FindConverter(description.Converter),
                ConverterParameter = description.ConverterParameter,
                Mode = description.Mode,
                FallbackValue = description.FallbackValue
            };
        }


        private IMvxValueConverter FindConverter(string converterName)
        {
            if (string.IsNullOrEmpty(converterName))
                return null;

            return this.GetService<IMvxValueConverterProvider>().Find(converterName);
        }
    }

    public class CustomBindingBuilder : MvxAndroidBindingBuilder
    {
        public CustomBindingBuilder(Action<IMvxTargetBindingFactoryRegistry> fillRegistryAction, Action<IMvxValueConverterRegistry> fillValueConvertersAction, Action<MvxViewTypeResolver> setupViewTypeResolver) : base(fillRegistryAction, fillValueConvertersAction, setupViewTypeResolver)
        {
        }

        protected override void RegisterBindingParametersParser()
        {
            var parser = new CustomBindingParser();
            this.RegisterServiceInstance<IMvxBindingDescriptionParser>(parser);
        }
    }

    public class Setup 
        : MvxSimpleAndroidBindingSetup
    {
        public Setup(Context applicationContext)
            : base(applicationContext, typeof(Converters.Converters))
        {
        }

        protected override MvxAndroidBindingBuilder CreateBindingBuilder()
        {
            var bindingBuilder = new CustomBindingBuilder(FillTargetFactories, FillValueConverters, SetupViewTypeResolver);
            return bindingBuilder;
        }
    }
}