You might want to check with TandemsEast or one of the other tandem-specialty dealers as I believe there are shorter (5.75 - 8.5" projection) stems available:
http://www.tandemseast.com/parts_frame.html
Obviously, some adjustable stems can be shortened a bit by hacking off an inch or two from the handlebar clamp boom that slides into the stem's base.
As for fixed stem lengths, I have ones in use that range from 85mm - 100mm (Ritchey WCS) so I know they're available as are the shim's needed to reduce the clamp diameter to fit your Burley's (26.6mm?) seatpost.
http://www.ritcheylogic.com/trpdfcharts/stems.pdf