For example (in open exchange package ks-iris-lib) :

/// list of data types exposing %GetSerial and %SetSerial
Class ks.lib.collections.ListOfDataTypes Extends %Library.ListOfDataTypes
{

/// serialize object
Method %GetSerial(force As %Integer = 0) As %String
{
    return ##super(force)
}

/// deserialize object
Method %SetSerial(serialized As %String) As %Status
{
    return ##super(.serialized)
}

Storage Custom
{
<Type>%Library.CompleteCustomStorage</Type>
}

}

There is a limit to string length, as explained in the documentation.

To convert a binary stream to a base64 encoded character stream, you have to call the function in a loop using a buffer. Due the way the function works, the buffer length must be a  multiple of 57.

Here a sample implementation, from ks.lib.stream.Utils class in the ks-iris-lib package available on Open Exchange.
 

/// encode stream data into encoded using bufferSize (due the implementation of $system.Encryption.Base64Encode, this must be a multiple of 57)
ClassMethod Base64Encode(stream As %Stream.Object, Output encoded As %Stream.Object, bufferSize As %Integer = 5700) As %Status
{
  #Dim sc as %Status
  #Dim ex as %Exception.AbstractException

  #Dim len As %Integer
  
  s sc = $$$OK
  try {
    throw:((bufferSize#57)'=0) ##class(%Exception.General).%New("buffer size must be a multiple of 57")
    $$$TOE(sc,stream.Rewind())
    s:'$d(encoded) encoded = ##class(%Stream.TmpCharacter).%New()
    s len=bufferSize
    while 'stream.AtEnd {
      $$$TOE(sc,encoded.Write($system.Encryption.Base64Encode(stream.Read(.len),1)))
      s len = bufferSize
    }
  }
  catch (ex) {
    s sc = ex.AsStatus()
  }
  return sc
}

 Using the second approach  : edited down to 39 characters 

ClassMethod ascii()
{
 f i=32:1:126 w:$t(ascii+1)'[$c(i) *i
}