I personally think if enevlopers ever make it in to the game then it should concentrate more on the effect they have and less on the doubling of strength that they get in SFB which is what people seem to fixate on. That is just a means to an end.
They are harder to shoot down, but wear your shields down rather than blast through them. In SFB their damage spreads around and hence only does about 1/3 the standard damage to any given shield. If you shoot someone with envelopers it will take 2 or 3 decent ones to go through the shields, unlike a normal plasma that can blast through in one go. They can find the weak points (the weak shield facing away from you) and do internals through them. They also require a good chunk of extra power, which can't be pre loaded like photons can, on top of the normal expensive arming cost.
Not fully though out, but:
Only applies to G, S and R. They fire at normal strength but only take half damage from phasers. If they hit a ship with shields they are multihit 2, but unless the target had boosted shields they do 1 internal hit per weapon (not AD) as they find a weak point and burn through it. If the target had no shields at the point of attack they do normal damage (D6). They ignore the klingon shield rule. They would be a power drain. Unlike other overloaded weapons they are not range restricted so overload doesn't really work. A slow ship could envelope all its heavy torps (choose 6" move), or go fast and envelope just one (fire one weapon system).
So a plasma cruiser with 2 Ss could potentially launch at 8" with both Ss that if they hit fully will do 16 damage (and extra 2 internal hits if the target hadn't boosted). Enough to take out a small ships shields. This is roughly about right. They take half damage from phaser fire, so a typical cruiser with ~6 bearing phasers would only take off about 2-3 AD, and still lose 10-12 shield damage, and as long as both still got at least 1AD hitting would still do 2 internal damage. Note the ACTA rule that requires that each weapon system is resolved separately, avoiding any issues about which plasma was hit etc.
Against targets with down shields they are still brutal without any extra damage boost, making them hard to shoot down is still quite potent. A standard 7AD R torp in ACTA will do 25 average hits and averaging 4 devastating crits it will pop all but the toughest cruisers.
On the face of it that looks like it would provide the expected sort of interaction, a way of taking out enemy shields which is harder to stop than standard plasma, whilst also leaking some damage through against shields. Against unshielded ships it ensures that your already brutal plasma is more likely to hit the target.
The power drain combined with the way plasma are all single line weapons keeps it hard to launch all as envelopers at close range.
There may be better ways, and I don't think that they are needed anyway, plasma is pretty good in this game as is. It will certainly be interesting to see how Hellbores are handled, but envelopers also need to handle the hard to shoot down aspect as well, and unlike Hellbores they do not put most of their damaged through a single weak shield, so they are not usually going to inflict the same percentage of internal damage against a shielded ship.