I don't think MacTerminal will do MacBinary or BinHex conversion on-the-fly - it will require a decoder on the Mac. MacBinary and BinHex were both methods devised for the transfer of Mac files (which incorporate a resource fork) via other platforms' file transfer methods such as the serial-line protocols (XMODEM, YMODEM, ZMODEM, Kermit, etc), or FTP. None of those protocols are resource-fork-aware, so MacBinary and BinHex encode the contents of the data fork and resource fork into a single data file that other platforms can store and transfer.

The result is that even if the source machine is a Mac, if you're using a non-Mac protocol (in this case, anything other than AFP) to transfer the file, it arrives on the Mac as a generic data-only file. An application on the Mac would be necessary to decode the file into its source again, with its resource fork (the resource fork contains the file type info). Newer applications, web browsers, etc, will often perform conversion automatically after a file transfer. But I don't believe any application in existence at the time of the 512 would do that.

I'm not sure what to suggest here to get BinHex or similar onto a disk accessible on the 512, unless you have another Mac with a 1.44MB drive accessible to you...