I know you can set up a tandem with a single-sided drive. My question is whether you can use older Shimano cranks (octalink or square taper) and put a third chainring on the outside of the large chainring (using spacers that John allen has on his tandem page). Would the Q factor allow it? 90s Campagnolo cranks? Could I fit a chainring on the inside of the front cranks and fit it to the stoker's cranks (on a front drive tandem since a chainring in back might hit the chainstays)? Any other suggestions so I could build my tandem for cheap(ish)?
Your only real constraint is travel limits of the front derailleur and picking bottom brackets that give you the ability to off-set the axle to support what ever configuration you choose to use for your same side drive.
There are just too many different ways to make this work. Frankly, if you're simply trying to do a tandem drive train on the cheap you can also cannibalize three standard cranks -- one triple and two doubles -- to come up with a cross-over crankset by simply re-taping the threads of the doubles to flip the rights to left and one left to right.
Putting a sync chainring outboard the outer ring works on some cranksets and not others. Only experiment will tell. Mountain bike cranksets are more likely to work because the crankarm swings wide early to clear smaller chainrings. Also, using narrow 9 or 10sp chain for the sync can buy an extra fraction of a mm for clearance.