I don't know if it has been considered using a Raspberry Pi as an interface between the Ti99 and other devices, or even as a device itself (emulating disc drives, RS232 interface, usb, ethernet, wifi, printer port, ??)
The Pi has several interfaces, USB, HDMI and Ethernet, but also some general purpose I/O's described here: http://elinux.org/RPi_Low-level_peripherals
I read a question on the TI99ers group on Facebook - how to connect a printer to your Ti99 if you do not want to invest in a PEB? Then you need a NanoPEB, connect it to your pc, run TIPrint on your pc.
Why not attach the Raspberry Pi to the NanoPEB, all you need is a MAX3232 and some capacitors to convert the 3,3v levels on the Pi to RS232 levels, then some linux/python/?? programming to forward what is recieved from the Ti to the USB port on the Pi.
You might even be able to use a WIFI dongle on the Pi's usb port as an interface to your printer (the Ti would then appear on your network :-))
I have not (at least not recently) checked what is needed to interface directly to the Ti99, but consider this as an input to a potential (cheap) solution to what might seem impossible.