If low dose aspirin is beneficial, would the dose need to be increased to get the same benefit for someone living at 10,000 feet in altitude?

No: altitude does not decrease the efficacy of low dose aspirin as anti platelet agent. As the name its interprets " low dose" Aspirin. So Aspirin anti platelet activity is not dose dependent nor altitudes make a difference.

No change needed: There is no data that would suggest changing the dose of ASA from low dose to higher dose due to altitude changes.