I assume the problem would be the other way 'round: text that says one thing to search engines, but then uses a custom ligatured font to say something entirely different to users.
I actually think the crawlers are more sophisticated than that. Recently I got an email from Google saying the text on my website was not big enough when browsing my site with a low resolution - which is why they choose to lower my rank on the results page. Clearly they have pretty advanced tests when it comes to accessibility of text, not only the content.