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?

-Nick