Both M206 Y10 and G10 P0 Y10 ought to work. G92 Y0 will not.
I appreciate the info from all, but David I think there are some issues with M206. I went back and played with M206 Y10 again and it does some wierd sh*t. It probably doesn't matter; I admit my need is an outlier. But, in case it is of interest:
What I commanded (except for the initial M206, everything was done using "Head Movement" controls)
What the head did (also, the "Machine Status" display always matched the physical head location, not an axis shifted location)–--------
M206 Y10
Home all axes head homes to Y= -10
Jog down to z=0 head moves to y= -20 while moving down (!?)
Jog Z+1 head moves up 1 and jumps to Y= -30 while doing so (!!??)
Jog to max Y Head moves to same physical y max as before the M206 (i.e. it doesn't really help me)
Jog Y+1 a couple of times (beyond the bed radius) the head jumps back y-10
Jog down to y min and a little off of the X axis normal movement
Jog Y+1 repeatedly to get away from the bed radius limit head moves in the X direction a little each time (!!??)
And other (repeatable) bizarre behavior.
But the bottom line is it doesn't move the axis globally. The head still obeys the M665 bed radius limit from the physical 0,0 center, not the axis-shifted center.
Using absolute coordinate gcode moves does remove the strange jumping around but the head still will go where I don't want it to go and it adds the confusion of the status display coordinates no longer matching the gcode coordinates.
Thanks!