I am tying to use this filter to update an existing entry:
However, in the documentation it says:
For my scenario it would be ideal if I could capture the Entry ID as part of the entry submission (in a hidden field). Is there a way to do this?
Alternatively, I could dynamically assign an entry ID prior to it being created?
If you are updating an entry, I'm assuming it already exists. If it already exists, you should be able to use dynamic population to populate the existing entry ID into a Hidden field on the form. Then you can use the example from the gform_entry_id_pre_save_lead documentation to update the desired entry on submission.
Here's an existing snippet that does the same thing: https://gist.github.com/spivurno/32e914d67723f89717be2904ce6234c4