My master JMS index server died, can I simply take the index directory from one of the slaves and make it the new master jms indexing server's base?

it will miss some of the latest updates, that's not going to cause issues but users might miss some results, or it could be a bit slower while HSearch hides results which don't exist on the database.

I'd suggest to reindex the database.

oh that's fine if it misses the latest updates. Those will be reindexed eventually. I just don't want users to miss out on searching while everything's being reindexed as it takes a very long time.