Perhaps other employers should also employ everyone half-time so that they get more information about their employees' market value?

If SIAI were paying Eliezer to be a "generic" programmer, then I suppose they could get a reasonable idea of whether he's a good one in the way you describe. Or they could just fire him and hire some other guy for the same salary: that's not a bad way of getting (where SIAI is) a middling-competent programmer for hire.

But it doesn't seem to me even slightly credible that that's what they're paying Eliezer for. They might want him writing AI software -- or not, since he's well known to think that writing an AI system is immensely dangerous -- in which case sending him out to work half-time for some random software company isn't going to give much idea of how good he is at that. Or they might want him Thinking Deep Thoughts about rationality and friendly AI and machine ethics and so forth, in which case (1) his "market value" would need to be assessed by comparing with professional philosophers and (2) presumably SIAI sees the value of his work in terms of things like reducing existential risk, which the philosophy-professor market is likely to be ... not very responsive to.

What sending Eliezer out to work half-time commercially demonstrably won't do is to measure his "effectiveness" at anything that seems at all likely to be what SIAI thinks it's worth paying him $100k/year for.

The most likely effects seem to me some combination of: (1) Eliezer spends less time on SIAI stuff and is less useful to SIAI. (2) Eliezer spends all his time on SIAI stuff and gets fired from his other job. (3) Eliezer finds that he can make a lot more money outside SIAI and jumps ship or demands a big pay rise from SIAI. (4) Eliezer decides that an organization that would do something so obviously silly is not fit to (as he sees it) try to decide the fate of the universe, quits SIAI, and goes to do his AI-related work elsewhere.

No combination of these seems like a very good outcome. What's the possible benefit for SIAI here? That with some (not very large) probability Eliezer turns out not to be a very good programmer, doesn't get paid very well by the commercial half-time gig, accepts a lower salary from SIAI on the grounds that he obviously isn't so good at what he does after all, but doesn't simultaneously get so demoralized as to reduce his effectiveness at what he does for SIAI? Well, I suppose it's barely possible, but it doesn't seem like something worth aiming for.

What am I missing here? What halfway plausible way is there for this to work out well?

I don't understand my proposal as related to signaling at all

What's the good reason to care about whether EY's salary is calibrated to the market rate, rather than/independent from whether it's too low or high for this particular situation?

it's about measuring EY's (and others') effectiveness, rather than taking it for granted.

I don't understand why SI (i.e., its board) shouldn't employ EY and MV full-time and continually evaluate the effectiveness of their work for it, like any other organization in the world would do.

The fact that both are costly is irrelevant, the point is that one has the potential to be vastly more costly than the other.