I am writing a secondary textbook to be used in public schools. In my state, the textbooks must adhere to standards, such as the Common Core State Standards (CCSS). It is typical for a textbook to contain an index for teachers to check where in the textbook which standards are used. I've used the following code to create such an index, if I simply place `\learned{}` somewhere containing the standard's ID, it creates a custom index entry, and `\placelearned` displays that index.
\defineregister[learned][compress=yes]
\setupregister[learned][style=sansbold, textstyle=slanted, n=1, pagenumber=yes]
\starttext
        \learned{CCSS+W.7.3}
    \input knuth
        \learned{CCSS+W.7.3}
    \input knuth
        \learned{CCSS+W.7.1}
    \startchapter[title=Standards]
        \placelearned
    \stopchapter
\stoptext

This prints an index like this:

Standards
c
CSS
    W.7.1    1
    W.7.3    1

My problem is my textbook also includes four separate workbooks. I need the index to show my publisher that I've covered all 300+ standards, so I need the index at the back of the textbook to also contain details of which pages in each workbook the standards are found. How can I combine the index so it refers to all books? Something like this:

Standards

c
CSS
    W.7.1    T 1, Wk.1 3-8, 12, Wk.2 8-9, 60-61 
    W.7.3    T 1, Wk.1 3-8, 12, Wk.2 8-9, 60-61, Wk.3 1-2, 54, 70, 90, Wk.4 99, 102, 104

...or possibly like this...

Standards
c
CSS
    W.7.1
    Textbook: 1
    Workbook 1: 3-8, 12
    Workbook 2: 8-9, 60-61 
    W.7.3
    Textbook: 1
    Workbook 1: 3-8, 12
    Workbook 2: 8-9, 60-61
        Workbook 3: 1-2, 54, 70, 90
        Workbook 4: 99, 102, 104

Is there any way to print an index that covers pages across multiple books?

--Joel