Is preferential hiring ever acceptable

Yes. Providing specific job construction executive job seekers a hiring preference is not inherently discriminatory. As long as the hiring actions taken adhere to construction employer policies and the policies are not discriminatory in nature, your preferential decision should stand up in court. An example might be those construction executive job seekers who apply on their own verses applying through a search firm where an employment fee must be paid.
The best practice is to play it safe. The EEOC can be brutal if they can prove the employer used protected characteristics (age, race, sex, etc.) to discriminate.
If your hiring policy spells out that certain job construction executive job seekers will be given greater consideration than others, make sure you go through the policy carefully to ensure all references to protected characteristics are absent, including any preference on union ties or political affiliations.
The key is to ensure that your hiring policy is written in a manner that bases decisions on the method of application, not on an applicant's personal traits.