Thanks for the reference, I looked at them quickly.
Separate bean files sound like a good alternative, however many of rentals share the same transactions, which would be tedious to manually separate them. I can automate to some extent, but spreading the source of truth into many locations made me wonder how I would reconcile / audit at a later point.
I am not able to find what the core really defines, but it sounds like if it supports expense/income can be a leaf node and there is a way to plug in and compute and produce the statement, that would work for me, with some work to write something on my own.
Ideally, I'd like to track expense/income per activity, it can be either a rental property, or a project, or some of investment activity. also there can be expenses/income for all/part of activities. And the final statement can aggregate them all together.