Just to tie together the comments & partial answers…
Current state of play [as of OS X 10.11.2] would actually appear to be "this can no longer be done".
Regarding the following on QLEnableTextSelection
… that one alternative will fix it whilst the other doesn't appears to be some kind of hopeful, yet unfounded, meme going round the interweb - people posting it on as received wisdom, without actually testing it.
Here is the method anyway… though it doesn't work for El Capitan...
Test whether the pref already exists, & if it does what its current value is, using
defaults read com.apple.finder QLEnableTextSelection
Then try one of these to set your new pref
defaults write com.apple.finder QLEnableTextSelection -bool TRUE
or
defaults write com.apple.finder QLEnableTextSelection -bool YES
Test again to check the value was at least written, should return 1
if present & enabled.
Restart the Finder if your result did change from 0
to 1
.
Empirically, YES
& TRUE
give the same result, as do NO
& FALSE