Rebel Xti - forcing flash to fire

I am currently using a Rebel XTi as a back up camera and I am looking for a way to make the flash always fire. I use it primarily for back lit photos and I need a fill flash.
I know there is a Night Portrait icon on the control dial but this does not force the flash to operate in bright sunlight.
I have found that if I set the camera to program mode and hold down the flash button on the left side of the camera body this forces the flash to fire all the time. This requires me to use two hands, one to hold down the flash button and one to push the shutter release.

I use the camera to shoot player huddles from the ground up where I put the camera on the ground and push it in the middle of the huddle and shoot up. I need a way to force the flash to fire without using two hands.
Here are some examples of what I am trying to do

For fill flash, use Av and press (once) the flash button to lift the flash.

My 30D will operate in fill-flash mode in P as long as the pop up flash is up, or my 550ex is plugged in and switched on.

It does seem rather odd to have to hold the button down the whole time.

Hermeto's use of Av makes a lot of sense either way, because the body has a mind of it's own in P, mindlessly following some predigested programme of Canon's making.

Lowner wrote in post #7514713
My 30D will operate in fill-flash mode in P as long as the pop up flash is up, or my 550ex is plugged in and switched on.

It does seem rather odd to have to hold the button down the whole time.

Hermeto's use of Av makes a lot of sense either way, because the body has a mind of it's own in P, mindlessly following some predigested programme of Canon's making.

Be careful when using built-in flash in P mode!

It will act as the fill flash only if the scene is fairly bright.
As soon as the ambient light falls below 10 EV, it will start acting as the main light and execute that predigested Canon’s program.

In Av (and Tv for that matter) mode it will always work as the fill flash.