Caching inProc BOs between invocations?

I have the following setup: REST broker calls inProc BO via BS. As BO initialization is time-consuming, I want to reuse the same BO during the CSP process lifetime.

I have tried the following approaches:

1. Set %session.Preserve to 1 (I would prefer not to use it as the BO object is not linked to a specific client)
2. Set $$$EnsInProcPersist to 1 (looked relevant)
3. Cached BS between calls

However, every time I call my REST endpoint, I get a new BO. Is there a way to cache inProc BOs in the CSP context?

To run the sample, import it into any interoperability-enabled namespace, start in.Production production and call GET test123/dispatch/0 (test123 web app is autocreated on production start).