Is there anyway that a sub-report of a report can pass any data back to the main report?

(This may be a dumb question but here goes.)  

I have a report with a number of sub-reports (SQL Server 2012).  The sub-reports have tables that can vary considerably in length.  A certain somebody wants the report to contain a table of contents at the beginning of the main report, containing the actual page number of the start of each sub-report, the page number generated automatically by SSRS.  (We customarily export these reports to PDF.)  

Everything I've read so far seems to indicate that it is impossible to pass anything back from a sub-report to a main report.  I was hoping I could update a shared data set using Custom Code within a sub-report but that appears to be a dead end as well. Other programmers within my organization have supposedly also spent time researching this but without success.

Suggestions?