Is there an available securities law exemption for the participant's interest in a voluntary, contributory ESOP?

The rule of thumb is that you do NOT have an exemption if either: employee elective or after-tax contributions are invested in employer securities, or if the participant has the right to direct the investment of part of his account in employer securities.

If only employer contributions (including matching contributions) can be invested in employer securities and if those contributions are mandatorily invested in employer securities, you have an exemption.

Obviously, this is a hugely dangerous area for a nonexpert, and you should have professional help if you are working with a plan with employer securities (if you are not yourself an expert).