app to manage symbolic links and move files automatically?
IF you have a Mac that runs 10.8.3 AND has room for an extra internal drive. (Sacrifice the DVD on older MacBooks &amp; MacMinis)  A DIY "Fusion Drive" will accomplish your "goal".

http://blog.macsales.com/17624-os-x-10-8-3-provides-fusion-drive-setup-option-for-non-fusion-drive-equipped-macs

It is not a simple or strait-forward process however, 
Made DIY fusion drive.... now recovery partition won&#39;t boot