From my understanding one still needs a wildcard /* up front,( but I'm not sure)

Anyway you forgot ; as separator in searchstrings.

I'll suggest just /*displaytype= it should cover all cases and only searchstrings, since = is meta.

I thought I read that Google has a page for testing rules online, but I can't find it right now. *

FWIW: I read at Google that adbots are not covered by * and must be listed explicitly.

But I suppose those are not our biggest headache.

On a side note: adding rel='nofollow' to links posted by users should help frustrating spammers.

Edit

*) See https://support.google.com/webmasters/answer/6062598?hl=en

Couldn't test it since it requires a log in.

Update

Forgot to mention, I tried finding displaytype nodes with Google and DDG and the nofollow rules seem to be effective on site:www.perlmonks.org.

Other domains seem to show cached results.