Is it possible to overload the ToBitMap method to render the encoded value as part of generating the bitmap?
MultiFormatWriter writer = new MultiFormatWriter();
BitMatrix result = writer.encode("CR01234567890", BarcodeFormat.CODE_128, 100, 100);
WriteableBitmap bitmap = result.ToBitmap(BarcodeFormat.CODE_128, "CR01234567890");
This method should generated the bitmap with the barcode and the encoded value i.e. CR01234567890 underneath it. In the case of say UPC-A the encoded string (as numeric values) would display in the appropiate format for this barcode type:
Example barcode images with encoded values displayed