if you wire the DC volts backwards (very likely) then there shouldn't be smoke - either a fuse blows or the equipment tolerates it - thus my use of a polyswitch.
Why not a diode?

I find you have to look beyond just the item cost of the pcb. The issue of labour (especially in testing) tends to outweigh the pcb cost. Every situation is different but I find engineers tend to have a myopic view of cost - sometimes cutting cost in one area increases the cost in others - it becomes a bit of a ying/yang sorta thing.

two comments
1) you are right in the saveing can cost money. One example: using a cheaper part cost several field engineering trips which we calculate at $585 each - some savings.
2) in an app like yours, it can be beneficial to locate the things likely to go poof on a separate board, that will save on replacement costs.

Erik