You may consider to send email to build requester if something abnormal happens. Something like below:
groovy:
import com.pmease.quickbuild.util.MiscUtils;
import com.pmease.quickbuild.entitymanager.SettingManager;

if (request.requesterId != null) {
def requester = system.userManager.load(request.requesterId);
def mailSetting = SettingManager.instance.getEmailSetting();
MiscUtils.sendMail(requester.email, null, null, "email subject", "email body", null, mailSetting, "text/html");
}

Has there been any improvement in this area since? We are looking for a way to validate the inputs to a build.

For text prompts. there is an option to specify a regex pattern, and for numbers I can specify a range. However, it would be good to be able to validate more fully, for example by comparing the input to a list of valid values (like usernames).

In my specific case, I have a configuration picker, but I want to fail if the user selects a configuration that is disabled, or is outside of a specific tree structure. Right now I am doing it in a pre-queue script, but as the previous poster observed, the error message shows only "Build request is ignored [because] pre-queue script evaluates to false.".

It would be awesome to be able to configure this message.

You may just throw exception in pre-queue script, for instance:

groovy:
if (...)
  throw new QuickbuildException("Something is wrong");

It will be displayed in UI.

Okay, this will do for now. I will submit an enhancement request for better input validation.