May an individual elect whether or not to participate in the Trust (like a cafeteria or 401k plan)?

No, there is no individual election to participate in the Trust.  Generally, either the entire bargaining unit must participate, or no one may participate.  The tax advantages of the Trust depend on the absence of individual election.  However, as discussed in part A1, you may carve out part of the bargaining unit to participate, e.g., new hires. These requirements are set by the IRS.