Answer by seeker for Caching/preloading files on Linux into RAM

vmtouch seems like a good tool for the job.

Highlights:

• query how much of a directory is cached
• query how much of a file is cached (also which pages, graphical representation)
• load file into cache
• remove file from cache
• lock files in cache
• run as daemon

vmtouch manual

EDIT:Usage as asked in the question is listed in example 5 on vmtouch Hompage

Example 5Daemonise and lock all files in a directory into physical memory:vmtouch -dl /var/www/htdocs/critical/

EDIT2:As noted in the comments, there is now a git repository available.