Hi John,
Yes, you can change the configuration files to have the DOS diagnostics run on timeout rather than the Bootable. In order to do this, you will need to insert you Multipurpose USB device into a Windows machine so you can access the configuration files. Once this has been done navigate to the \isolinux directory within the USB device. You will need to edit the following files using a text editor (I used Notepad ++):
live_486.cfg
live_686-pae.cfg
live_amd64.cfg
Upon opening each of these files locate the 'ontimeout' value and alter the statement after it to include 'dos_boot'. It will read "ontimeout dos_boot" (without the quotations) after the alteration in each file. This modification will have DOS launch upon timeout. If you wish, you can also edit the timeout value so that it occurs faster by altering the 'timeout' value in these files. The value is set to 200 (20 seconds) by default. You can change this to whatever value best suits your needs.
Please note, should the USB device need to be restored these modifications will not carry over. They will have to be manually implemented once again.
I hope this accomplishes what you are after. Please let me know if you have any questions or need any further help.
Thanks,
Channing