in following the venerable tradition "if everything else fails, look how Fanuc 
does it":

here's what I found on valid operations during feedhold on some Fanuc controls, 
I wasnt aware of those features, and I summarize them to encourage thought 
about this sore spot in linuxcnc.

during a feed hold you can execute:

1. a 'General Purpose Retract' activated by a button:
The tool is retracted along a predefined vector and feed (probably relative to 
current position)
The program is aborted.

2. 'Tool retract and recover': remove tool and return to machining
The tool is retracted along a predefined vector and feed (probably relative to 
current position) on 'retract' button
Manual or jogging can be enabled, the control will record this "withdrawal" up 
to 10 jogs moves
returning to auto mode and hitting the 'recovery' button: follow the withdrawal 
moves in reverse; 'cycle start' will now move the tool to the point where the 
'retract button was pressed'
There are provisions for 2) during rigid tapping as well.

3. 'Manual intervention and return'
after switch to manual mode, tool can be jogged away
after return to auto, and pressing 'cycle start', the tool will return to the 
point where it left off.
see p574 of b) for the reentry move ('Non linear interpolation type 
positioning')

4. 'Threading retract':
Feedhold would normally kick in after finishing the current thread and 
returning to the start point.
With this option, the tool immediately retracts with the same chamfer angle as 
programmed, and returns to the start point.

5. 'Retrace': while 'retrace switch' active:
finish current block, then run in reverse (up to ca 100 blocks back)
feedhold stops retrace, cycle start continues retrace
turning off the retrace switch switches to forward execution again

sources:

a) http://www.cnc1.com/News/customer-files/CNC_eNewsletter_Mar_2011.pdf
b) http://www.cnc1.com/images/customer-files/Fanuc_Manual_Handle_Retrace.pdf
c) http://www.cnc1.com/images/customer-files/Fanuc_Retrace.pdf

- Michael
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