I have an Eotech EXPS on my AR, which has seen a number of rounds shot through it without any issues. But a .223 recoils much less than a 45-70!
However, considering these sights are mounted by the military guys on several platforms, including full auto weapons, which are treated fairly roughly, I would suspect they can take a fair bit of recoil/abuse.
Perhaps send an email to Eotech and ask them the question to confirm.