Is it possible to put a bottom bracket meant for a 73 mm bottom bracket shell into a shell meant for a 68mm shell with the requisite 5mm of spacers?
Yes!, but you mean a 2.5mm spacer (2.5mm on each side = 5mm)
If the bottom bracket has a shoulder on the drive side, you only need one 2.5mm spacer (Some bottom brackets ship with a spacer, so you can use them for 68 or 73) - No shoulder means 2.5 mm of threads showing on each side.
Quoted message said:
I know physically it would be possible, but do you run into structural and/or safety concerns (not to mention the wider Q factor).
Done it many time before. Only the body is 5mm longer. Spindle is the same length, same Q.