No problem, thank you for elaborating!
The only way to access values that are contained within a custom expression node is through the node's outputs.
It's possible to have several outputs in your Custom Expression node through using the Qualifier parameter of the added Inputs. Please refer to the image below:
We do have a visual issue with the ports and names not showing correctly on the node itself, but they're working as expected. By setting the Qualifier to Out, the new output port continues to stay as an input port, but any values connected to that will be ignored. Also, the parameter name that you set for the Out port might not be correctly displayed on the node, which should also be ignored as it will be working internally.
Please let me know if this helps, I'll be happy to elaborate!