reset/reboot SM5100B

I have a project based on a SM5100B -- It's the Sparkfun shield form-factor board:

In my application I want to avoid power-on boot of the GSM module and have it boot only when my sketch wants it to.

Does anyone know if this module can be programmed to NOT boot at power-on, so that I can ask it to boot through software command later? What commands would be suitable?

I can wire the reset of the module to be held low after power-on, until I raise it under control of a digital output. Any idea if it is fair play to hold this module in reset for hours on end?