Re: Can we get a sub-string from the Product "containerName" attribute value in an OIR, or add a string attribute to the Product and use that instead in the OIR ?

Have you tried leaving off the StringConstant lines and just use the containerName attribute?  Like this:

<Value algorithm="wt.rule.algorithm.SubString">

     <Attr id="containerName"/>

     <Arg>0</Arg>

     <Arg>5</Arg>

</Value>

It works this way with the number field of the part (when not autogenerated).