UPDATE.
After a lot of rejections, one gunsmith explained why no one wants to work on an Infinity: it's the replaceable breechface. It makes cutting the slide for an optic very complicated and easy to screw up. As a result, no one wants to touch it (after looking at it, I understand why.)
Instead, I've ordered a rear sight replacement plate for an SRO. It's not quite as low as a direct cut, but it's close, it's a cheap part ($55), and I can install myself. It's a little ghetto but I'll shoot this for a season and if I like it, will just have a complete slide made; or maybe a new gun. Thanks for the input, everyone!