Altering a trajectory mid move

So I have this project, putting a new control on a laser welder. It originally had a Anorad control on it which died about 12 years ago. After it died the company that had it started looking for a new control to put on it. They chose "OpenCNC" by MDSI. Its a real expensive software package, about $11k for 4 axis. It works with a variety of pretty generic IO boards as well as a couple of the serial protocols like Mechatrolink. That is what was chosen for this machine and they bought Yaskawa SGDH series servo sets and mechatrolink modules to handle IO.

The reason they went with OpenCNC (Which is anything but) is that is is pretty open to modifying how it works through its API. The feature that is needed for this laser that the other controls could not do is alter the trajectory while making a move. Sometime the seams in a part are just not straight so the toolpath needs to be "nudged" one way or the other.

I am having one hell of a time getting the loop closed in the software. The software sends the velocity command to the drives and receives encoder position back and is supposed to complete the loop in the software. Its not working so well... So at this point I am looking at one of two things. Option 1, use the internal position control in the drive and use step/dir signals to run the machine. Option 2 is use a 5i25 and a 7i77 and drive the analog input.

So my main question is, can I do what opencnc would allow, shifting the path a bit either using a button or maybe a mpg? Would this be easier to do with the loop completed in linuxcnc and maybe somehow add or subtract from the encoder counter?

Actually even the step/dir hardware/software gets feedback, so the offset could be inserted there. You would want to run the MPG through the limit3 component
so that you dont exceed the velocity/acceleration limits (and you will have to have enough velocity/acceleration headroom for the manual offset)