How can I monitor an installer as it modifies the filesystem on Yosemite?
This not application specific (it will show all) but you can find the one you are looking for.

iosnoop

This “traces” disk I/O execution live. Each time a disk I/O completes, a line of output is printed to summarize it, including process name and filename details:

Example:

sudo iosnoop

UID   PID D    BLOCK   SIZE       COMM PATHNAME

  503 54079 R 286522800   4096 Google Chrome ??/Cache/data_2