I know this is a couple years old, but just to clarify, you can carryback capital losses in an AET with a late filed 104(13.2) election AND provided the trust had no taxable income in the year you are applying the loss against.
For example if the year you want to carry the capital loss back to, the trust also received eligible taxable dividends from a Canadian corporation AND had allocated expenses against that dividend income, you will find the trust had taxable income related to the dividend gross-up (but likely paid no tax due to an excess dividend tax credit). In this specific scenario, a 104(13.2) election will be denied because the gross-up created taxable income in the trust. You can avoid this trap by not allocating expenses against eligible taxable dividends.