You can create a TypeConverter, something like this:

public class SomeClass
{
    public int Key { get; set; }

    [TypeConverter(typeof(LookupConverter))]
    public Guid LookupKey { get; set; }
}

public class LookupConverter : TypeConverter
{
    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        if (destinationType == typeof(string)) //property grid will ask for this
        {
            var key = (Guid)value;
            return ConvertGuidToDescription(...) // TODO: implement this
        }

        return base.ConvertTo(context, culture, value, destinationType);
    }
}