I think we should not only make competitions who creates the best game or demo, but join efforts in creation of the universal diagnostics ROM that could help to identify common problems in our MSX machines and report them even if there's no audio and video outputs from a computer. Sadly, my proposal to create such a project was not taken seriously.
This is exactly what I've been thinking for a while and makes a lot of sense to implement. And I would like to start coding something like that once I find the time..
The thing with MSX architecture is that it's very difficult to a) disable internal bios from cart and b) make the CPU boot arbitrary rom at $0000 from cart, like C64 dead test does. Some Philips machines have a diagnostics switch that enables such a thing, but it's far from universal if it only works on those, and the Philips diags cart already exists.
It would make more sense to approach it like the Amiga DiagROM, an eprom that you can plug instead of the main bios rom. It's feasible to write test code that doesn't use RAM, no STACK, so basic diagnostics could be done. Out of band debug output is difficult since RS232 isn't standard on MSX. Screen flashing could work, but then VDP and analog out needs be working. Maybe a small parallel port dongle with LEDs, since every MSX has a parallel port?
Btw - I almost forgot to say that I really like your Leds cart Alexey, it would be useful to diagnose things like stuck bits on databus 
Thanks for the nice feedback.
Yes, replacing the ROM with a custom one would be a very good idea. The ROM could run from power on and it could do in-depth diagnostics of the machine and output the data onto the special cartridge with a decoder and its own screen. So the code in ROM will send the results to the cartridge using some simple protocol, the cartridge would decode the messages and display them on the screen. This way it would be possible to see the test results and errors if the video output doesn't work. At the same time that cartridge could have voltage sensors as well as a small microcontroller to sense the clock, verify the proper Reset signal action and maybe analyze other signals.
Creating such solution is not a trivial task. And not everyone will be brave enough to disassemble his computer, replace the ROM chip and do the testing. That is why a simple diagnostics cartridge like this one with the special ROM could be a good alternative.
