I'm not sure we have the ability to calculate that, let alone calculate it reliably and completely. We'd only be able to tell the ETA for the current step, assuming that data is exposed. Like, if the par2 commandline app we have can tell us ETA, then we should be able to scrape it, but we wouldn't at that point be able to guess how long unrarring will take.
And stuff like post-post-processing scripts would be completely out of the question unless we can get everyone to expose standardized ETA data.