There is a perfect way to do what you want, especially since you don't want the computer next to you however it does involve getting another piece of equipment - a foot switch which send "space bar" to the computer. You also need to set up BIAB to start with one of the spacebar options shown on the first preferences page and select "pause until midi or key received"

This gives you complete flexibility to play whatever length of intro you want, at any tempo you want then bring in BIAB in strict tempo at just the right time. I do it this way for almost every song although I am using a keyboard. I love this method because it allows me to "throw away" the tempo for the intro, this can be very effective.

Foot switches are not expensive and can be easily programmed to sent "spacebar"

Tony