The most simplistic way to utilize hamlib is via simple shell-script calls from AllStar. 
Here is a super simple -working- sample that demonstrates changing to channels 1, 2 and 3 on a 706mk2g.

-c0x58 is the  model-specific configuration parameter value

Add this to rpt.conf in the desired functions stanza:
[functions]
*921=cmd,/usr/local/bin/rigctl -r /dev/ttyUSB0 -m 311 -s 9600 -c0x58 h 1
*922=cmd,/usr/local/bin/rigctl -r /dev/ttyUSB0 -m 311 -s 9600 -c0x58 h 2
*923=cmd,/usr/local/bin/rigctl -r /dev/ttyUSB0 -m 311 -s 9600 -c0x58 h 3

So, DTMF commands *921, *922, *923 change to channels 1, 2 or 3. 

Note *that the path to rigctl and path for the serial device must be correct.*