Reading from a TFile is that is being written to is supported to some extent.
////////////////////////////////////////////////////////////////////////////////
/// Read the linked list of keys.
///
/// Every directory has a linked list (fKeys). This linked list has been
/// written on the file via WriteKeys as a single data record.
///
/// It is interesting to call this function in the following situation.
/// Assume another process1 is connecting this directory in Update mode
/// - Process1 is adding/updating objects in this directory
/// - You want to see the latest status from process1.
/// Example Process1:
/// ~~~{.cpp}
/// obj1.Write();
/// obj2.Write();
/// gDirectory->SaveSelf();
/// ~~~
///
/// Example Process2:
/// ~~~{.cpp}
/// gDirectory->ReadKeys();
/// obj1->Draw();
/// ~~~
/// This is an efficient way (without opening/closing files) to view
/// the latest updates of a file being modified by another process
/// as it is typically the case in a data acquisition system.
Now it is likely that this has not been stressed test where the writing frequency is high compared to the reading speed.