I am not a lawyer, so I can’t speak to the legal aspects, but I want to tell you to stop worrying about this 
It’s okay if ZIPs need to be changed after first being submitted, and both organizations will consult our counsel as needed. The purpose of a draft ZIP is to be clear, not perfect. In other words, when writing a ZIP, it’s practical to avoid obvious and well-understood legal issues, but no one expects you to settle the things that have to be settled by lawyers.
Block rewards to miners are distinct from a dev fund.
The rest of this comment is my personal thoughts.
There are two layers of choice.
First, users choose whether or not to use Zcash, the software and the network, at all. That choice is always in play, since people are perfectly free to stop using Zcash, or never start using Zcash in the first place.
Because of this, even a mandatory dev fund can be seen as voluntary. If you don’t want to contribute to a Zcash dev fund, but it’s an intrinsic requirement of mining or using ZEC, then you can just decide not to. You can fork Zcash, or convert to one of the competitors. No one is forced to hold ZEC or transact with it.
As @acityinohio wrote:
[U]sers are always free to fork the cryptocurrency or exit the ecosystem, although not necessarily with equal resources at their disposal. In either case, users cannot take the network effect with them — exchanges, services that integrate Zcash, stores that accept Zcash, etc. — unless they leave en masse.
If everyone knows that running Zcash software, participating in the Zcash network, means that they are contributing to a dev fund, then everyone is contributing voluntarily.
I waffled back and forth about that for a long time, but I’ve decided now. You always have the right of exit; that’s one of the radical things about cryptocurrencies.
Like @NealJayu said on Twitter:
I’ve heard the #ZcashDevFund referred to as a ‘tax’. Not remotely close and totally compatible with libertarian theory: it’s a voluntary deed-restricted community with homeowner dues which go towards the development of the community. Nobody is forced to move in, but they do. Why?
Anyway! There’s another layer of choice if miners are involved in directing where the dev fund goes. Either they decide between N recipients, or between N recipients and burning. (Or at least those are the options that people seem interested in.)
(By the way, I still haven’t decided, just personally, whether Zcash should have a protocol-based dev fund at all. But my concerns lie elsewhere. I plan to share my full personal stance on what Zcash should do — not the pros and cons of various options, but my “vote,” so to speak — after submitted ZIPs have gone through some debate.)