Does Maryland allow deduction of property tax on out-of-state personal use 2nd home?

Yes, it's deductible for MD taxes. It doesn't matter where the property is as long as it's an ad valorem tax.

I'm not sure what meets the ad valorem test, but this is a school district tax that the county assesses based on their appraisal of the home's value.  I think I saw in the Fed instructions that a school tax is deductible provided that the tax is assessed on the home's appraised value.

That's right, if the tax is assessed based on the propery's value then it's deductible. Just like for the "tax" portion of a vehicle registration fee.

Thanks.