I did check the source forge project page for HamLib. I didn't see any support mentioned for any radios in the Yaesu Line prior to the 800 model series. Also,I did post a support request to the project mailing list asking if such support is already on board or, if not, can it be added. As of 20 minutes ago, I have yet to see moderator approval for the post. Still, until I get the FIF-232c interface box here, I won't be able to test if it is supported. I have also sent an email to Yaesu customer service (yes, they still exist) asking for the specifications for the command set used to communicate with the radio. I hope to see some kind of response this week.
Anyway, thanks for the suggestion.
since you mention Linux, "hamlib" comes to my mind. It's a library offering an unified control interface (from the programming point of view) to various ham radio transceivers. Part of hamlib is a small command line program called "rigctl", which can be used to easily set and query parameters of a transceiver.
While "rigctl" doesn't seem a popular choice for most users, it may be just the right thing for you, with fully textual in- and output. And I suppose you'll use most of the functions of the FT-757 (e. g. the VFO knob) "manually", and you mainly need a way to read out the current frequency from the transceiver.
Hamlib is available in the package repositories of most current Linux distributions, not sure about OS X.
Maybe this helps a little...