Poor man's Tempilaq, rather.

Seems reasonable enough. Only works if the affected spot gets hot enough to melt and coalesce the vapor, of course. Might not work for PCB shorts (low resistance, too much thermal mass?).
Any kind of temp-sensitive phenomenon will work, like, you could deposit a fog of... some liquid, and watch where it evaporates more quickly. Like foggy windows, but, water might not be suitable for obvious reasons, alcohol has too low surface tension (forming a film rather than droplets), maybe a slightly heavier alcohol or light oil would do, not sure. Oh, or alcohol with something dissolved in it, possibly rosin again, but another organic compound (or even inorganic salt) that crystallizes well, leaving a white spot as the film evaporates, could show in a similar way. Or a low melting wax, to a similar end as the rosin. Of course you then need to clean off that residue before continuing (unless it happens to be soldering-compatible, like rosin).
Tim