Think I'd suggest avoiding low pressure sodium lamps for applications like this.
While the arc tube in the 18W lamp is probably small and light enough that with some consideration given to the lamp mounting arrangements, it would survive...there are a number of drawbacks.
The 1700K colour temperature and zero colour rendering index are two obvious factors. Also worthy of note are that these lamps take a significant amount of time - in the order of ten minutes - to warm up, producing hardly any light at all initially. They also don't fare well when started frequently, as starting's hard on the electrodes.
The diffuse discharge is also quite difficult to direct accurately if you're looking for anything less than a full flood.
Vibration in use may have the negative effect of causing the liquid sodium of some lamps to travel to the lowest point in the lamp, in spite of the normal retention methods, leading to starvation of sodium vapour at the higher points...with as small a lamp as the 10 or 18W, I wouldn't have thought this was really an issue.