As in the question - how can I install a persistent backdoor with Metasploit on a Linux target?
Any persistence modules for Linux would be found under than "Local" exploits here: https://github.com/rapid7/metasploit-framework/tree/master/modules/exploits/linux/local
Or in the Linux Post Modules: https://github.com/rapid7/metasploit-framework/tree/master/modules/post/linux
Or in the Multi Post Modules: https://github.com/rapid7/metasploit-framework/tree/master/modules/post/multi
I didn't see any that perform persistence, but like @d33tah said, usually persistence in Linux is simply editing the crontab to run something or installing SSH keys so that you can get back in. Or you can go the route of Kernel rootkits: http://www.securityfocus.com/tools/category/65