It works as long as you don't have any zaps of your own available.
Or if we don't care how many you have! If we are just willing to have this suck up non-free zaps.
Again this system obsoletes zaps. Their only value, in this situation, is to let you make a level easier if you are having trouble winning a fair fight (well some games there are multiple opponents, but even there, it's plenty doable).
Example: I have 20 zaps available and I want to apply 3 on Base 1.2 (which is already balanced at 7C/3E each). The default behavior is that I use the 2 free zaps and 1 of my own. If the extension sets freeZaps=0, it would look like I had used up 3 of my own zaps and now had 17 left. But either way, the server calculates that I have 19 zaps left, and that's the number that gets sent to the client when I open the next adventure level.
Acceptable.
Of course, Salvager could just ignore how many zaps the server says you have. For example, it could balance the starting hands and then disable further zapping by just telling the user that he has no free zaps and no zaps of his own. That would require some explanation, lest a user buy a flask of zaps and then get irate when it looks like he still has no zaps available, but it probably wouldn't be too hard to implement.
Yes, ignore, set free zaps to zero. The goal is to maximize the joy of playing these levels (shy of them actually making all of the intended improvements); that joy is maximized by having zero free zaps. Your nonfree zap total is not relevant, those are just fun-sapping things that have no merit beyond letting you deal with being bad at beating a level (in this situation; with no mod they let you have an awful pointless game on your way to grinding to get promos, rather than an unwinnable game). You get as many tries as you need, the bots are not great, eventually you will win that level; and if not you can get some nonfree zaps at that point (for free as it happens, since they just cost playing dominion) and make it easier.
It just does not matter that your supply of hard-earned nonfree zaps goes down, that's the way I see it. Maximize joy.
Alternatively, Salvager could try to adjust the number of zaps the server says you have. In the example above, the client goes on to Base 1.3 and the server reports that the user has 19 zaps available. Salvager could adjust for the two free zaps on Base 1.2 and tell the user than he only has 17 zaps available. But making that adjustment consistently over multiple sessions could be challenging.
I would stick to the lower-effort straightforward zap-wasting fix. We are still hoping that these guys, who looked promising back in January, will somehow get around to fixing the adventures, at which point this will all no longer apply.
If you are worried people will be sad to lose nonfree zaps, you could make this change optional.