I really have a hard time figuring out why such a big capacitor is used on the board, I have been working with the SIM900-module, and the requirements for the power supply isn't as crazy as that: http://zapro.dk/public/PDF/AN_SIM900%20Reference%20Design%20Guide_V102.pdf Look at Page6 in the PDF.
I've made a simple Breakout-board for the module during hardware development, and i've found absolutely no problem with the stability of the VBAT-suppply.
In the end i'm going to power the module off a protected Lithium-cell (Nokia cellphone battery) with a suitable Lithium-charger IC. Such a protected pack is foolproof - any abuse will make the electronics in the pack shutdown and prevent anything nasty. It wouldn't be any worse than leaving a Cellphone or iPod in the car, plugged in.