I wouldn't mind such kind of block, however it'd be important to change how shields and HP system works for the sake of it. I would need to look for the suggestion, but I regarded it in recent post:
applying and deriving HP only from certain blocks so ships with a hole blown in the side will have to protect its 'guts' from further attacks and a bunch of guys in a boarding party will be able to enter and damage internal components of a ship without shields and armor magically inverting itself to protect every computer and a pot-plant.
Currently, with shields protecting interior of a ship and armor granting durability and being based on all blocks instead of just external/hull ones, I am not sure how much good exploding reactors would do - they would be horribly hard to balance, since some ships with still potent shields and armor would simply shrug off their explosion and some would be totally eradicated with no chance to avoid their fate, often based on a freak chance of 'it had a few seconds to recharge' which shouldn't matter much in case of something important exploding inside of the very ship.
We need the non-hull interiors of ships to be weak so wrecking stuff, be it through critical reactor meltdown or - maybe one day - actual saboteurs and boarding parties would make sense. We also need the shields and HP modified so once exposed the reactor won't be protected by the rest of the hull/recharged shields - because if one has to go through all the armor and all the shields anyway, the whole exploding reactor thing is just afterthought since the ship most likely already lost and is ripe for pickings.
Other than that, I'd rather leave energy generating blocks just that. While I understand the sentiment of greater customization through changing weapon etc power based on what the blocks are connected to, I think it would be just a gimmick not worth another long process of balancing energy, balancing weapon output, balancing it in regards to shields and hull etc. What would need to be balanced is the current power gen blocks - they should scale harshly enough to not be viable to maintain very high drain well on their own - otherwise people will just keep using the 'old' generators with new ones being just small boost providers for times of greater need, not actual basis of ship's power. And then again, there wouldn't be much worth exploding.
Edit: And yeah, regarding points raised above - I wouldn't mind thr grouping and gain of dimensions increase the power as it happens with current power gen blocks, but with a curve in scaling making really low amounts of power till the generator gains certain size and with the rise in size, the size of explosion should rise considerably, for the actual 'whole Death Star goes boom' potential, though with perhaps about 15-20 seconds of warning before explosion, for the sake of core jetissoning/evacuation pod deployment and delightful 'uh-oh' impressions.