You need the schema to deduce the meaning of the data contained in such encodings, you don't need it to safely parse the BER stream. Only slight complication is

You need the schema to deduce the meaning of the data contained in such encodings, you don't need it to safely parse the BER stream. Only slight complication is that you have to handle indefinite length constructed encodings of unknown tags, but that is simple, because you know that the encoding is constructed in that case (first, because the P/C bit of even unknown tag has correct value and secondly, because indefinite length encoding is always constructed).