I think your plan sounds like a really good one. I've done similar things in the past when we needed a solution that was pretty obviously NEVER going to occur to anyone at school.
Basically, you have to come up with a plan that you think will work, make sure that it's the path of least resistance for the school, and make them think that they've got more input into it educationally than you do. All at the same time. (Let's just say that I can hum a few bars along with you.

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To make it sell a bit better, you might clarify how this WON'T make more work for any of the (overburdened) classroom teachers or administrators...
As in, WHO does the instruction if she misses a pretest? (I know that you want to do what you can to prevent that outcome in the first place, but I'd include that contingency plan just to make it more complete and cooperative-seeming.)
I'd probably make it clear that "self-study" is more or less what you're thinking there, and that YOU will take on any additional "at home" instruction that the teacher recommends, or some such platitudinous statement. That way you aren't saying that the teacher is useless, or that you think you can do a better job (even if that may in fact be true), but you're also not even remotely "making more work" for him/her, either.
It's a narrow bandpass, that.
On your last point, that is, the fact that you don't care whether the teacher really interacts with your DD re: the non-normative material she covers; I'd make that one sound more sympathetic. Whether you believe it or not is more or less immaterial, because I've found that a teacher's level of cooperative spirit rises exponentially in direct proportion to how "busy/overworked" a parent suggests that the teacher is. Frame that one as "understanding" that you're already asking for SOOOO MUCH extra for your child... and that you really don't want to add even more to the "workload" (implying that it must be crushing), so here's your idea... is that still too much work for the teacher to manage?
Is this helping?