I think in fact it would be a lot of work to implement (and test enough to be confident about it). As folk have said, it wouldn't happen often enough to be worth the time, even without considering whether there is historical precedent.

This does raise a somewhat related point, in RTW (and in MTW2 maybe, can't rememeber) some units had the ability to swim across rivers, while others did not. I wonder if there will be any swimmers in ETW or whether bridges and fords will be the only way across for everyone?