Alternative to AlwaysAppendSearchDomains on Yosemite?
I found the answer as I posted this:

To enable mDNSResponder, edit /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist and remove the lines 

&lt;key&gt;Disabled&lt;/key&gt;
&lt;true/&gt;


Then add -AlwaysAppendSearchDomains to the arguments list further down in the same file:

&lt;string&gt;_mdnsresponder&lt;/string&gt;
&lt;key&gt;ProgramArguments&lt;/key&gt;
&lt;array&gt;
    &lt;string&gt;/usr/sbin/mDNSResponder&lt;/string&gt;
    &lt;string&gt;-AlwaysAppendSearchDomains&lt;/string&gt;
&lt;/array&gt;


Finally run 

$ sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist
$ sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist


You can then verify mDNSResponder is running:

$ ps ax | grep -i dns | grep -v grep
10456   ??  Ss     0:00.44 /usr/sbin/mDNSResponder -AlwaysAppendSearchDomains


Subdomains of search domains can then be resolved.