Problem with RS232 communicate

[eric233]

Hello everyone,
Following up on my previous discussions regarding RS232 communication drops in VirtualBox 7 (Oracle ticket #18668). After analyzing the code, we identified and fixed several bugs in the serial port handling (UartCore.cpp, DrvHostSerial.cpp, serialport-win.cpp). Our tests on several pieces of industrial equipment show a clear improvement — no communication drops since deploying our patched version (testing is ongoing).
Our modifications affect exclusively ring-3 components (VBoxDD.dll and VBoxRT.dll) — the kernel driver VBoxSup.sys is not modified.
To deploy this patched version across the rest of our fleet, we're constrained to keep Secure Boot enabled and testsigning mode disabled (internal security policy, as the VMs run legacy, no-longer-maintained operating systems).
Question: is there a build configuration or hardening mode that would allow signing only the ring-3 components with an internal certificate (without a Microsoft EV certificate), while keeping the official Oracle VBoxSup.sys and the Secure Boot trust chain intact? Or does the build certificate verification mechanism necessarily require consistency across all components, kernel included?
Thank you in advance for your insight,

Version: VirtualBox 7.1.10