While I wait for confirmation that indeed, as I suspect, the 7800 has no way currently to get characters into the onboard PSK and RTTY utilities except via a directly connected USB keyboard, I have been looking for a suitable device to emulate a USB keyboard and drive key codes on the USB port form an RS232 (or other USB) port.
A device that looks promising is the Vetra Systems 331 USB protocol converter.
Such a device would certainly fit the bill, and allow me to use the internal utilities' presumably excellent radio integration with some home brew software that will do all the fancy text handling conveniences, including substitution macros and logging.
I shall probably order one of these (they're relatively cheap, and get even cheaper in OEM version without packaging). This also gives me the motivation to get cracking on my software, knowing that by hook or by crook I shall be able to use it as intended.