The positional embeddings are static, so the max model_input_length is more a reflection of how the model was finetuned than a fundamental limitation.
I got the params from: https://github.com/google-research/pegasus/blob/master/pegasus/params/public_params.py 13
For some of the saved state_dicts on S3, I simply removed the embed_positions.weight to allow passing max_position_embeddings=ANY_INT to from_pretrained allocate more at init.  Let me know if you try that and it fails and I can delete the embed_positions.weight as needed.