Integration Services License requirements
If users are not accessing the 2016 Instance itself, there's no need to licence them for CALs to it, just need a licence for itself for production purposes - Server or (in this case, avoid) Core. 

It is unusual that you would use a Server licence without CALs, Microsoft would probably be interested on an audit... but as you note, your users are on 2014 CALs accessing their 2014 Server Instance, so would be covered for that (assuming no pass-through or multiplexing going on). However, as at least one user will need to use SSMS or VS etc to manage the 2016 SSIS packages or indeed server, I'd suggest a 2016 CAL for them.

Disclaimer: I'm a Microsoft Licensing Sales Specialist and sell MS Licencing online in UK, any advice may be given freely but ultimate responsibility is of the licencee.