You can use the "Q" key to open all outer doors (or as many as you wish). Set up your shots the way you want. Then, when ready, just shoot off all the tubes as fast as you want to - as good as a salvo I'd say.
An aside question? Is the reason for the no salvo firing due to the air cycling of the tubes? Reading
http://www.maritime.org/fleetsub/tubes/chap1.htm#1B there is a lot of air being exchanged within the boat, and I thought that was the reason you could not fire salvos - you had to give the air and displacement water time to cycle.