Orienting a rotary axis to be perpendicular to direction of travel. Possible?

So this is a bit of a off the wall question... I'm working with a friend that is wanting to make a CNC spray painter for painting the numbers on the highschool football field.. He has the placement situation figured out, and is planning on using an X/Y frame from an old CNC router to move the paint head. The paint will be supplied by a Wagner Airless sprayer, with paint flow controlled by a solenoid valve. All this is really pretty simple until we hit the gotcha moment....

The spray tip is a fan shaped pattern. If you control with simple X/Y moves, you'll have the same effect as writing with a calligraphy pen, super weird looking letters. For it to work correctly, the fan shaped output of the nozzle must always be at a 90 degree angle to the direction of travel. Not so hard really, we can use a live swivel coupling to allow the nozzle to rotate and a stepper to control it as a C axis in LinuxCNC. It would be easy to incorporate this into the G code for straight line (G1) moves. The problem comes in when we try to do arcs, like for a number 2, 3, 5, 6, 8, 9, or 0... The direction of travel is not in a particular direction, but rather an arc.... I have no doubt LinuxCNC can handle the coordinated movement of the various axis's. The issue is, how to get the input into it to knno what moves to do? Is there anyway in HAL or otherwise to have a control that keeps a rotary axis at a fixed angle to the current direction of movement? Or will I need to CAM this out using tiny G1 moves with slight corrections in the C axis for each move? 

Next question, for turning the paint on an off, on a machine with good latency figures running a MESA interface, say I turn the paint on using a M8 before the move and off using an M9 after the move, how much of a delay do you think I'd see? (Electronic delay, not the delay in the solenoid valve) I know my old Haas VMC has a decent delay between lines when processing things like coolant on/off, spindle on/off, etc.
 

siemens 840Dsl has a function called "Tangential control". But I'm afraid linuxcnc doesn't have this similar function
 

My suggestion is: post process the G code that controls the movement of the paint, discrete the non-linear movement into micro segments, and specify the corresponding rotation axis angle according to the tangential direction of the micro segments. This should be a faster and more convenient implementation method.

And the delay should be very small，30-50ms perhaps. Industrial machine tools like Haas usually use PLC to control the cutting fluid switch, so the delay is longer.

Option 1 - Tangential orientation can be handled in a post processor.  Here's an example on github for a Fusion 360 tangential knife. github.com/jejmule/PostProcessor

Option 2 - External offsets.  I can't give you a cut-paste recipe, but there are some clever folks here who have done some remarkable things with external offsets.  Like lathe cam turning.

linuxcnc.org/docs/master/html/motion/external-offsets.html

External-offset-per-angle could approximate a continuous C-axis tracking. Probably good enough for spray-painting grass:
linuxcnc.org/docs/devel/html/man/man9/eoffset_per_angle.9.html

Option 3 - get or make a different tip. i.e. circular pattern.  Either spray pattern (fan or circular) will have lighter regions at the edges so some experimentation with pass overlap would be in order.

Assuming you're using a power painter with built-in cup to get around the paint supply tube twisting up... Have a rotary union for the electrical or do you plan to unwind the cord after a few turns?

Or something else clever up your sleeve?

You've sparked my curiosity, 'cause I'm now imagining a robotic gantry with a 10-gallon reservoir that can crawl across a field...or a street, or a gymnasium floor, or a warehouse floor...

Or any other expensive place that spends lots of money on large-format sign painters...

You can use M62 and M63 for ON or OFF digital output synchronized with motion.

linuxcnc.org/docs/html/gcode/m-code.html#mcode:m62-m65