To be honest, I think it's a little too risky.
First, you'd have to bluff a scarf,, in order to lure out Ferrothorn, which can be difficult. Furthermore, Fire Gem means Palkia loses a lot of power, as compared to Lustrous Orb, Life Orb, Choice Specs, etc. Also, Ferrothorn is almost always used in rain, so the calculations are 252 SpA Fire Gem Palkia Fire Blast vs. 252 HP / 208+ SpD Ferrothorn in rain: 292-344 (82.95 - 97.72%) -- 25% chance to OHKO after Stealth Rock, which admittedly is pretty good, but if you screw up, or they switch out, you waste your Fire Gem on most likely a resist in the rain.
I'll test it out when I have time though, but Ferrothorn is already 2HKOed by Fire Blast in rain, so it really comes down to how skilled a player you are. I suck, so :/