Answering myself to post a follow up question 
Is there still work going on to include Kostal in Modbus/Sunspec support?
First of all, should this topic stay here, or may I use this info to create a feature request?
I know there are a few hurdles but if I can do it, it should be a piece of cake for Victron, right?
I guess the biggest blocker for most people to do further testing at the moment is to get a connection to the inverter.
To make this easier for people I would suggest making the port configurable instead of having it fixed to the default port 502.
That alone should be enough to make most moder Kostal devices visible to VenusOS.
At the moment this can only be achieved by some sort of port forwarding dirty tricks that might be to complicated for most to even give it a try.
For the non-yet-working limiting on the Kostal device.
Kostal published their specs on their website.
KOSTAL Interface description MODBUS (TCP) & SunSpec with control information
It has just recently been updated (June).
This plus more can be found on their Service Portal .
Section 3.1. describes active power and reactive power control.
It seems as simple as writing the desired power as a value to address 832 and for very large systems to include the scale factor on 833.
Like I described here.
Sorry if I am too pushy here.
It is the pure excitement that it is (almost) working and should be made available to more people.