Are there any plans to add a constant voltage feature on the re:load pro?
It's on our todo list, but not near the top. We welcome source code contributions, though!
do you think it would need to be in hardware or just a slower software loop changing the target current?
That depends on your use-case. Software constant voltage support is certainly possible, though doing it in hardware would obviously offer better response time.
What are you wanting to use it for?