question about MPE and AAL5 over satellites

I am an engineering student, currently working on my thesis in cooperation with Alcatel about security on the ethernet layer in satellite networks. The network I have considered in my work uses PPPoE encapsulated by MPE in MPEG transport streams for the forward link and encapsulated by AAL5 in ATM for the return link. If you insert security fields, the possibility exists that the payload of the ethernet frame becomes larger than the allowed 1500 bytes. My question is now whether it is possible to modify MPE and AAL5 to allow frames with payloads larger than 1500 bytes to be inserted?

It would help me a lot if someone could answer this question for me since I studied the MPE and AAL5 standard and did not find an exact answer for this question.