If anybody (including Epyon / PaulStoffgren) is interested in working on this to produce a non-specific-board centric solution I can provide a microSD image for the RPI 3 with the complete SCADA software. You will need to sign a simple NDA to get that.
Well, I definitely am interested in fixing any problem with Modbus RTU on Teensy 3.x & LC. If you give this library a try on Teensy and something about Modbus RTU isn't working, I will try to fix it. While I can't make any guarantees, I do have a long history of fixing problems with many Arduino libraries.
That said, I do believe there's a very good chance the edits I recently made resolve all the issues, making this library work on Teensy at least as well as it does on the 8 bit Arduino board it was originally meant to support.
If you encounter any issues, I'm going to need you to be specific and post non-proprietary test code here on this public forum. I'm not going to sign any NDA.
I have no intention of testing this lib on other non-Teensy hardware, other than perhaps running a copy of Uno or Mega which is supports. Sorry, I can only dedicate my engineering time to supporting Teensy hardware.