I've started looking at this. Mainly because the update command (using the search endpoint) isn't working for me, but modified_since seems to work fine for me.
One thing I'm getting hung up on is how would you build the batches? The search endpoint returns one set of results for all types. If you switch to using modified_since wouldn't you have to make a request for each type? At that point I'm not sure how you incorporate 'max-pages'. There's no endpoint that uses modified_since but returns results across all types, correct? If I'm understanding correctly batches are being processed and tracked by 'last_user_mtime'. So if I run a batch on the first 10 pages, those pages are sorted by user_mtime, and when the batched update finishes up to max_pages, 'archivesspace.latest_user_mtime' is updated so you don't reprocess the ones you've already processed. But that wouldn't work across multiple types using the iterator, right (because user_mtime wouldn't be sequential across all types anymore)?
Just FYI, we decided to go a different route for this (created a proxy for our ArchivesSpace instance). If this isn't useful to anyone, feel free to close.
Adding a patch for this since I have an immediate need for this. It's the same change made to the merge request above.