Would the assets in a secular trust be subject to the creditors of the participant? We're talking about an ERISA plan...obviously one that is not tax qualified.

If it is subject to ERISA (covers employees) and pays retirement income or defers income until termination of employment under section 3(2) then it is protected from alienation under section 206(d) regardless of whether it is a qualified plan. It is assumed that the trust assets are for the exclusive benefit of the participants. Its no different than a plan which has been disqualified under the IRC but is still subject to ERISA.