The strap adapter is going to be located on the back of the Pebble and it will provide access to the Pebble pins (GND, VCC, Data).
The GSM module will be placed in the upper strap. It will be powered via the pins from the strap adapter and its serial will be connected to the Data connection as described in the Pebble smartstrap documentation.
The whole smartstrap assembly should be designed and programmed to consume very low power. The whole device should maintain it's battery life (that's one of the amazing things about pebble, isn't it?). That's why I chose to go on the route of a microcontroller + GSM chip. Usually, a GSM chip uses small current, but it has some power saving features that can be enabled. This is where the microcontroller comes to play: we can program the uP to put the GSM chip and itself to sleep when not needed. The data link from the Pebble can be used both as interrupt line and as data line for the uP. So, for a smaller cost and smaller hassle we can connect directly the GSM module to the Data line at the cost of battery life, or we can connect the uP to the Data line and the GSM module to the uP to get better battery life.
The GSM antenna should be designed to integrate with the whole assembly as sleek as possible. It could be integrated in the strap, or with the GSM module, or on top of the strap, or even on top of the watch.
On the lower strap (the one that is closer to your body when you look at the watch on your hand), optionally, I would want a small group of buttons (4-6-9 buttons). I want to develop a sophisticated Pebble app that will send various messages and those buttons will help to do this very fast. Also, they can be used for other applications, not necessary GSM related.
The jack and the micro USB sockets could be added anywhere where is a place and don't embarrass. The jack should be connected to the mic and speaker pins of the GSM module. It will be used to plug a handsfree and talk to persons when making calls. The microUSB will be used for charging the Pebble.
The optional battery could be added to maintain and extend the battery life of the whole device. It should be assembled in such a way to permit the current to flow into the Pebble and work as an auxiliary power supply. The microUSB must, also, charge this battery.
Some good components for this project could be made from RePhone kits, or Xadow modules.
I will add some drawings of the whole device. Don't expect something eye appealing.