How do you program milling a dome. Is there a canned cycle, like a pocket clearing cycle, that I could add a z feed to? Maybe a helix cycle with a slow z feed?

Is this a full or just part of a hemisphere you want to machine.
I have a program that will generate a helix around the outside of a hemisphere.
Advise diameter and were you wish to start and finish. Also the dimensions of the cutter you intend to use.

there isn't a cycle that I know of.

I have done it in turbocnc and Emc2 using loops.

Here is how I did this in Emc2
http://www.electronicsam.com/images/woodcube.jpg
http://www.electronicsam.com/images/KandT/axisubuntu.png

#1=1.25 (size of box)
#2=.0625 (cutter radius)
#3=.125 (size of box bars)
#4=5 (degrees of resolusion)
#5=#4 (COUNTER)
#6=[[#1/2]+#2] (ACTUAL RADIUS OF CIRCLE - CUTTER RADIUS + RADIUS)
G01X0Y0F10
o140 do
#8 = [[SIN[#5]*#6]*SIN[45]] ( X AND Y POSISION)
#9 = [0-[[1-COS[#5]]*#6]] (Z HIGHT)
G1X[#8]Y[#8]F3
Z[#9]
G18 G02 X[0-#8]Z[#9]I[0-#8]K[0-[#9+[#6]]]F5
G19 G03 Y[0-#8]Z[#9]J[0-#8]K[0-[#9+[#6]]]
G18 G03 X[#8]Z[#9]I[#8]K[0-[#9+[#6]]]
G19 G02 Y[#8]Z[#9]J[#8]K[0-[#9+[#6]]]
#5=[#5+#4]
o140 while [#8 LT [#1/2-#2-#3]]
G1Z1
X0Y0
M30

there are some other examples and info here
http://wiki.linuxcnc.org/cgi-bin/emcinfo.pl/emcinfo.pl?Oword