With todays modern electronics, I could not see this being a problem..
The only negative I can see happening (apart from the reduced power (RPM/torque) from the motor), is a possible excess build up of heat in the ESC and motor from with less efficiency because of the lower speed..
Is the auxiliary function something that runs all of the time like a hydraulic pump, or something that only runs occasionally, like a hoist setup..??
Another option you could try if you want to reduce the speed of the motor and keep the ESC settings at 100% is to reduce the supply voltage.. You could do this by using a 1S (1 cell) LiPo out of a 1/12 racing car, or if your still using NiCd or NiMh batteries, make a 4 cell pack to run this ESC and motor.. The only thing that you will need to do to make this work is remove the red wire from the receiver plug on your Timbuk2 ESC when connecting it to a common receiver (which you should have already done if this is a second ESC anyway)..
Here is an example of a 1s LiPo battery..
http://hobbyking.com.au/hobbyking/st...r_Lipoly_.html
Another option would be to fit a reduction gearbox between the motor and whatever it connects to, to reduce the speed.. Here is an example of the HPI unit
http://www.hpiracing.com/en/part/87634