What can you do with a Parallel Port ?
Haha this is awesome stuff really. would have loved to see that in action 
Happen to have some video of it? 
EXPERIMENT 1: I used like in 1992 the parallel plugged to a like DAC resistors (IIRC was GND - 1k + signal OUTPUT+ 1k + 1k + 1k + 1k + 1k + 1k + 1k + 1k ), some like that for outputing WAVE to the sound channel, it is very EASY to do, can fit all in the LPT plug, so you don't needs to hard modificate the MSX itself and piggy the output to the sound channel.
As I wasn't capable to record, the idea was to use a MIC and monitor in an old full analog TRC osciloscope (Yes I have one ugly shity triggering as analog can be), and to see how are the vocals real soundwave, and then to try to draw those waves using HEX numbers feeded in DATA sentences of a BASIC's programs, the idea was to see if I was able to replicate the human voice.
I was capable of get VERY CLOSE results to the real voice, but noisy as hell.
EXPERIMENT 2 with MSX: later, I was looking a way to RECORD digital lines, without access to a real digital osciloscope by its very expensive price, and I was needing to decode the communication between a microcontroller and its CMOS 8 pins DIP memory, (in a radio-cassette for cars) with anti-thief system where you enters a code that can be from 1111 to 6666 to unlock, I was trying to visualize for the code being readed by the microcontrolled from that fucking CMOS RAM chip. It is basicaly works like a serial, so basicaly I was looking on DATA & CLOCK, I use the JOYSTICKs inputs for that, create a litle basic program that acts like an osciloscope with the capability to SAVE multichannel digital data syncronously, and then to study those strips, screen 2 & just BASIC is enought for that low frequency communications.
EXPERIMENT 3: Merge, 8 joysticks inputs as digital sensors, plus 8 outputs (LPT PORT) to control everything you want, converting the MSX as a big programable easy MICROCONTROLLER 
YOU CAN DO 3D printing with that IF you want, it has enough inputs as outputs to control everything, but I did that to control lights in a dance (not the professional equipment with professional protocols, no, it was homemade lamps controlled with some electronics, in a piggy proto board (now all that is garbage in some box)).
Hooked up a relay board to my 'experimental' parallel port cartridge.
I used a ULN2803 to protect the parallel port and a step-down converter to power the relay board with a external power supply.
Here is how it looks like:
And a movie of it in action:
Got my empty cartridges today and made a 2nd version of my 'experimental' parallel port cartridge.
Everything in 1 cartridge :D
The Jumperblock of 8 jumpers (D0..D7) on top can be used to redirect a datapin to a Led. Without the jumper the datapin can be used for example a Relay (see my relayboard) or a servo or an arduino or what ever you want to control.
The 2 other jumpers redirects Busy & Strobe to Ground.
The 2 other pins are 5V & GND (not to be jumpered :p )
On the side there is a power connector to supply 5V, the input voltage can be between 6V & 30V, a step down transformer, transforms it to 5V. This Powersuply is decoupled with a ULN2803.
