Installing El Capitan from a bootable DVD
I've created bootable DVDs (DVD+R DL, specifically) for Mavericks, Yosemite, and El Capitan, and used them without any problems to install on various MacBook Pros. Thus, the OS X installer can't require write access to the installer's volume (as of 10.11.x anyway). Creating an El Capitan installation DVD is slightly different than previous, but a user 'tywebb13' on MacRumors is who I've followed each time I've created these DVD installers. He's posted the Terminal commands (including short explanations for each) to create a bootable El Capitan DVD (or just the ISO if you skip the last command).

http://forums.macrumors.com/threads/el-capitan-bootable-dvd.1923931/page-2#post-22407115

NOTE: Some of the posts in that thread refer to installing them in virtual machines, which adds confusion. But 'tywebb13' is consistently focused on optical media installers for actual Apple hardware, and I've always followed his instructions to success.