WOW! I'd call this the most credible surprising argument for truthfulness I have seen in a long time. Figures it's from Eliezer. Score in our years long argument over the strength of the prior for truthfulness.
Note though, that to be a good idea this would have to be done very sensitively. Also, the girl would have to be awfully rationalistic. I'd default to the position that any girl who isn't already poly is fairly unlikely to be a good candidate for this sort of argument, accompanied by a firm assertion that rationalist guys should not restrict themselves to poly girls.