Indeed my understanding is that it should not require crazy hardware requirements - it's a serial protocol clocked by the programmer.

Here's an Arduino project to program CPLDS/FPGAs using XSVF files https://github.com/wschutzer/xsvfduino

I'd say that the cheap programming cables are abundant and already compatible with the vendor tools so there's not much motivation to reinvent the wheel.

TMS is hard to manage. High data rates are exactly the issue.

Any MCU can bitbang JTAG at 100kHz. Try doing it at 30 MHz sustained.