how to load parameters from other aplication?

What is the external program language?

I think that an O-CALL needs to be to a G-code subroutine rather than an arbitrary executable. Perhaps a custom M-code will work better:

linuxcnc.org/docs/2.7/html/gcode/m-code.html#mcode:m100-m199

Not elegant, but if the external routine is a shell script then

halcmd setp motion.analog-in-00 123.456

Will give a value that G-code can read with M66.

There is a Python interface that has the capability to write to G-code parameters. I think you would still need a G10 command to load the values into the coordinate system. Python can run G-code through an MDI interface, but I don't think it can do so during program execution.

linuxcnc.org/docs/html/config/python-interface.html

That doesn't document setting G-code parameters, but there is code here that does it:
github.com/LinuxCNC/linuxcnc/blob/master...n-stdglue/stdglue.py

I think it requires:
But some experimentation will be needed to get the incantations right.

I have done some experiments, and have determined that my ideas above probably won't work.

Setting G-code analogue inputs will work, but can hardly be called elegant.

I think your best bet is a remapped Python routine, but one of the more advanced ones:

linuxcnc.org/docs/2.7/html/remap/remap.h...onfiguring_remapping
(look at "Advanced example: Remapped codes in pure Python" )

From what I can gather you probably want to use the Python code to send acommand, but I would need to do some more experimenting myself to work out exaclty what is required to do that.

Sternly: Are you still reading this? I am prepared to try to sort this out, but there is no point if you are no longer reading.

If that is _all_ that the G-code does, then perhaps you could consider not running a G-code program at all.

It is relatively easy to put a Python program in charge and that could send movement commands through the MDI interface.

A HAL component (probably user-space) could be created with 3 HAL pins that could be net-ed to the motion.analog-in-00 / 01 / 12 pins to be read-in with M66.
linuxcnc.org/docs/2.7/html/hal/halmodule.html

Maybe the trick is to start from the other end. What measures the required positions?

Niko wrote:

I've been thinking about this a bit and wonder if the G codes can be remapped to take parameters from the outside .

Yes, this would be a neat way to do it. Probably using Python for ease of integration.
Using remapping you also get access to the emccanon library, which lets you do lots of potentially useful things.
It isn't well documented, but you can use all the things here:
github.com/LinuxCNC/linuxcnc/blob/master...74ngc/canonmodule.cc
I think you also get to use the stuff here:
linuxcnc.org/docs/2.7/html/config/python-interface.html but I am not 100% sure about that.

A much more elegant but labor intensive one would be to make a custom GUI but then I'd still need remapped G codes for that

For a cartesian machine you could do everything in HAL and simply send joint positions directly from a GUI to stepgens or PID components. (ie, no G-code and not much of LinuxCNC). For a SCARA you need kinematics, though. You almost certainly can access kins without having G-code and Motion, but it's probably too much trouble,