I am working on TIPI, with ElectricLab. It is an open sourced work in progress interface between a TI-99/4A and a Raspberry PI, so that we can provide a 'hosted' filesystem for the TI and leverage networking capabilities of the Raspberry Pi. The initial designs are prototypes. ElectricLab has a pair wire-wrapped logic boards using 7400 series logic to address memory mapped IO with the Raspberry PI. He has built some really cool software off of that to get two 4A's interacting.
FYI: This is the source code of the PALEA5 program. It just sets up the palette and then quits.
VDPWD EQU >8C00 * VDP write data
VDPWA EQU >8C02 * VDP set read/write address
START LIMI 0
* F18A Unlock
LI R0,>391C * VR1/57, value 00011100
BL @VWTR * Write once
BL @VWTR * Write twice, unlock
LI R0,>01E0 * VR1, value 11100000, a real sane setting
BL @VWTR * Write reg
* Load color paletttes