Get the ring insured in case of a mishap, both before the propsal and thereafter.

I actually checked my wife's ring on our surprise engagement trip. A smart idea? Perhaps not, but I didn't want somebody helping themself to a very lucrative "gift" from my carryon bag while I was in an Ambien-induced slumber on the transatlantic redeye.

If you do carry the ring aboard the aircraft, I'd suggest a lock for whatever compartment the ring is placed in. Further, you might want to consider placing it in a less conspicuous container than the telltale jewelry box that probably originally accompanied it.

Good luck with the proposal!