Zinger

It would stop rust but here's what I know about using WD40 on micrometers:

The solvent in it evaporates leaving a film that's attractive to dust and it's not recommended for either micrometers or clocks (from what I've read) for that reason.
And I can verify the mess that it leaves inside a mic because I've taken the spindles out and seen it. You'd do better using motor oil on a micrometer.

Now if you were to coat a chain with it before waxing, however, I'm not sure how that might work. If a film stayed under the wax that might work out. But I suspect it would interfere with the parrafin's bonding to the chain and if you spayed it onto the waxed chain the solvents in the WD40 might break down the wax coat. It would be interesting to hear from anyone who tried it. I'm still considering whether I'm going over to parrafin or not. It's not like I'm not happy with what I've got but the cleaner chain and less maintenance is appealing....The rust not so much.

When I first started cycling I remember using WD40 on chains without anything else (as well as motor oil) and, as I recall, it did pick up more grit than the Starrett M1 that I replaced it with as does generic aerosol silicone to some degree (but not so much as the WD40). It also didn't work so good at lubricating by itself. I've never tried mixing WD40 with other lubes though.