You can override type descriptions before using types with the TypeDescriptor class, passing it a custom type descriptor.

It's a bit convoluted and you want to make sure you scan types and properties before you configure type descriptor or you may run into stack overflow. (TypeDescriptor.GetProperties has a noCustomTypeDesc parameter but it seems ineffective with my .NET 9 tests...)

Here is a sample TypeDescriptionProvider that should do what you need from assembly B (.NET Core code):

Data obj = new Data();

// configure a custom type descriptor for the Data type
var td = new DataDescriptor();
TypeDescriptor.AddProvider(new DescriptionProvider(td), typeof(Data));

propertyGrid1.SelectedObject = obj;

Sample classes:

public class DescriptionProvider(ICustomTypeDescriptor descriptor) : TypeDescriptionProvider
{
    public override ICustomTypeDescriptor? GetTypeDescriptor([DynamicallyAccessedMembers((DynamicallyAccessedMemberTypes)(-1))] Type objectType, object? instance)
        => descriptor;
}

public class DataDescriptor : CustomTypeDescriptor
{
    public DataDescriptor()
    {
        var properties = TypeDescriptor.GetProperties(typeof(Data));
        var list = new List<PropertyDescriptor>();
        foreach (PropertyDescriptor property in properties)
        {
            if (property.Name == "Text")
            {
                list.Add(TypeDescriptor.CreateProperty(typeof(Data), property, new TypeConverterAttribute(typeof(CustomConverter))));
            }
            else
            {
                list.Add(property);
            }
        }
        _properties = new PropertyDescriptorCollection([.. list]);
    }

    private PropertyDescriptorCollection _properties;
    public override PropertyDescriptorCollection GetProperties(Attribute[]? attributes) => _properties;
}

public class CustomConverter : TypeConverter
{
    public override bool GetStandardValuesSupported(ITypeDescriptorContext? context) 
        => true;
    public override StandardValuesCollection? GetStandardValues(ITypeDescriptorContext? context)
        => new(new List<string> { "hello", "world" });
}