ETOPS implications
It's not simply engine failures during volcanic ash exposure that needs to be considered.
Is it not possible that a period of significant ash exposure may have implications for engine in-flight shutdown (IFSD) rates in the weeks and months to come? Is it safe to assume that engines that have spent significant time operating in ash environments will have the same IFSD rates as those that have not?
ETOPS certification depends on statistical assumptions based on IFSD rates. If ash exposure leads to changes in IFSD rates then these assumptions may no longer be valid. The fact that both engines on an airframe will likely share the same recent history of ash exposure is also relevant.