Earthquake insurance, deductible as a percentage, better to take lower coverage amounts?

All the companies I've contacted so far have a separate deductible for earthquake claims...deductibles...are always a percentage

this is industry standard; you will not find any exceptions to this.

does the deductible as a percentage make it advantageous to purposely select lower coverage totals?

No. The insurance company will not sell you a policy with a limit that is less than the value of the home and they have software tools to ensure the value makes sense.

More importantly, the reason earthquake deductibles are so high is that catastrophic events (earthquakes, hurricanes, etc.) tend to either completely destroy or completely miss a house; don't think of it as the company making you cover the first $25k, think of it as the insurance company paying you $225k if an event happens instead of $250k.

on my current home insurance I have $230,000 in personal property coverage (separate from dwelling coverage). However, I do not have $230,000 of personal property inside my house -- it'd probably be closer to $80,000.

Contents coverage is almost always set at a percentage of the dwelling limit - usually around 20%-30%, so I'm guessing you have $750k to $775k in dwelling coverage. Talk to your agent or company about options to lower that, although you're not likely to save a lot of money doing so since most of the earthquake premium is based on the value of the home.

Wouldn't it make sense to lower my personal property coverage to $80,000 (or even slightly below it)? Such a change wouldn't much affect my annual premium, but it would significantly lower the threshold at which my policy will pay out.

Read Your Policy: Most earthquake deductibles are based on Dwelling Coverage * Pct Deductible, not (Dwelling + Contents) * Pct Deductible, although this may vary. If it is Dwelling Coverage * Pct Deductible, then it is obvious that lowering your contents coverage will not decrease your deductible.