Does the second-level cache have the intelligence to not issue an update statement when an unmodified object (which was previously loaded from cache) is reassociated with a session?

you mean using update()

no, it is not that intelligent.

Are there any alternatives?

We have a multi-page form workflow and want to avoid an update when the user has not modified data and clicks "back" or "next"

1). If you are using ehcache, you can tune timeToIdleSeconds and timeToLiveSeconds parameters for each cached class, for all other supported caches I think there is the same configs.
2). Another solution using of HttpSession or Statefull Session Bean to save intermediate data, but don't put very large data structures into HttpSessions.
3). some kind of onEdit javascript/event that marks the input like dirty and needed to reupdate.

Wouldn't it work if I load the object to be updated from cache, apply the changes (which could be none) and perform call update() within the same transaction? The object loaded from cache would not be dettached in such a scenario and it would be considered equal when update() is called. Is this a correct assumption?

You don't need to call update for attached objects:

It is OK scenario.