EtherCAT driver for a 6 axis robot

Fine.
Now, if I were you, I would create an xml file that linuxcnc can display. - I think it's definitely needed

Some of the steps / guides here are proving informative

sticky in this section
forum.linuxcnc.org/ethercat/45336-etherc...-step-by-step#237098

And also
gist.github.com/Bouni/8b4532d0bdf012bd83c65d3eb62f8aa2

hmm, what for this case is needed is an direct cyclic control of the joint motor amps, like cyclic synchronus position or velocity mode. without an inserted controller from the robots motion system.

The joint indexs that TheRoslyak marked in his picture are only feedback information from the robot to ethercat. controlling at this firmware is reached through the motion command block with the underlaying motion que. And this sadly is not rt controllable....

EtherCAT interface appears to support MotionCommandID with six arguments (for each joint?). Here are two - many others available. See table 7 in programming manual. "List of MotionCommandID numbers"

And I did not say that these registers are needed for control.
For control need move command (for example 1 - is line 2 - arc 3 - spline and etc)
And sub index ( is x ,y, z ,etc)
In any case, if you do it through linuxcnc, Need combinations of mux, dmux and etc

The only "cyclic" modes (velocity mode) are

MoveJointsVel
MoveLinearVelWrf
MoveLinearVelTrf

These can be sent evey cycle with MoveID=0, SetPoint = 1. To stop the robot send SetPoint=0

Would a velocity mode with position feedback from each joints encoder be such a bad thing? Robot timestamp,  World Reference,  Tool Reference and Robot Base Reference are available data on the TxPDOs in various data forms.