I am afraid that this is technically impossible, but if there is any way how not to loose all expensive and blood payed undead minions due to the crash (moreover, usually in the middle of the quest or DM event), it will be very helpful.
Would it be possible for when a necromancer dies/crashes/logs, that his minions turn wild similar to how unleashing them works? Then possibly add an ability to regain control of unleashed undead?
Or would this all be too powerful you think?
I have to admit, I empathise with this. Not to mention that it would sort of ruin the triumph of finally beating the vile necromancer if it happened due to a simple crash and relog :(
Very awkward to make work properly, I doubt it'll happen. There are simply some things that one has to bear with.
Only way I can think to make it work is keeping track of them through some sort of log and re-summoning them with the same bonuses when the player reconnects.