Sure, in principle it can be extended to Seq2Seq tasks…quite easily, I assume. It’s just not obvious how it effects the batch sizes. If you want to ensure that all inputs sequences have the same length (e.g., 12) and all all output sequences have the same length (e.g., 15), the likelihood to have enough item pairs for each existing combinations of lengths might be very low.
On general, for a large dataset, it should be not too bad, I think. One can argue that when an input sequence is of length X, then the length Y of output sequence should mostly be within the range of X. But I haven’t tested it yet. The very few times i dabbled with Seq2Seq models, I’ve used batches of size 1.