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
TRUE give the same result, as do