Waiting out a siege is a poor way to deal with invaders. I find that the best way to deal with enemies is to use the "hallway of doom" technique. This is where you have a long, twisty path that invaders full of death dealing devices that leads to your fort. They mention it on the wiki under Defense Guide.
Here are some of my favorites.
- Lines of Cage traps. You can later setup a "death pit" where invaders become "target practice" for your marksdwarfs. Easy to setup and useful in the early game.
- Carnival shooter. Where you have a large room with a switchback path that a number of marksdwarfs can watch over and fire at their leisure as the invader passes.
- Drowning locks. Again use the switchback, but setup some floodgates to both seal the room and open passage to a water source. Invaders drown, but you have to figure a way to drain the room. Can also be used with magma.
I have found that a siege could last for years and is generally better to take care of quickly. Always give invaders a path to your dwarfs, but make it so long and dangerous that only giant armies can make it through. Simply add a floodgates at the final choke point and with a flip of a switch, you can seal in the invader.
I have not tried capturing a siege in a way that keeps my dwarfs safe, but doesn't end the siege... I wonder if it will prevent another siege from occurring?