Your question calls for knowledge of Windows (DOS) batch files. You would need to make a bootable disk containing a batch file that will 1) mount the TrueCrypt voume and 2) then run the desired AV app.
Batch files have fallen into disuse, you might want to ask your Dad :)
First, search the internet for your favorite method of creating a bootable USB drive or CD for your version of Windows.
Next, you will need to create a batch file named autoexec.bat . This is a text file that you could create in Notepad, the extension .bat indicates it is a batch file and the reserved name autoexec.bat tells Windows to run this file at startup. A batch file is merely a list of commands such as you would type at a Command Prompt.
You can find TrueCrypt command line syntax at:
You can find command line syntax for your preferred AV product by searching the internet or your AV provider's site, for example for Sophos:
You might also want to put copies of the TrueCrypt and AV program executables on the USB drive or CD.