How can you set up a free, universal OS X-wide ‘autosave’ script?
Dealing with the same problem myself (especially needing effective autosave for excel for mac, which is most definitely broken in Mavericks) - I just started the "ForeverSave 2 (version 2.1.2)" trial on my Mavericks-running 2011 15" mbp and it seems to both force a cmd+s save of the excel document I'm working on and also save a copy to its library.  This does lead to some excess clutter but it has options to 1) keep the total number of backups for a certain file less than X amount (you specify amount), or 2) to automatically delete backups older than X number of days.  It does actually force a cmd+s, which is great - I tested several times by typing something, letting the document sit slightly longer than the ForeverSave auto-save time window, force-quitting excel or turning off my computer, and upon re-opening the file my new data was still there, without me having ever manually saved.  It only does this when the document is the active window, however so if you type something and then go start working on another document, it will not auto-save that first one (unless the autosave timer just happened to go off in the split second between you finishing typing and going to another window).  Only really seems like a problem if you make the autosave window very long (i.e. I could image if it was set to 3 hours I might work for two hours, navigate to another window, and then have a problem that quits excel, with no autosave ever happening) - to compensate I just set the autosave window to 2 minutes, and am having Foreversave delete any backups that it has after they're a week old, so it's library stays small.

Silly that we have to pay $20 for software to do something so simple, but at least it seems to work.