Upon testing EZP-26182, which required to create a custom block attribute for an ezflow block, we have not been able to find procedures for this on the documentation, hence from QA's point, documentation for this should be created.
We however managed to come up with a way to use by analysing existing settings files and adapting, such as, in the lines:

https://github.com/ezsystems/ezflow/blob/master/packages/ezflow_extension/ezextension/ezflow/settings/block.ini#L70-L84