I have a client doing this, while it's not perfect, it DOES work. In his case, instead of running a cleaning cart in the LmLc slot, it's just a dead head ( which I'm sure will cause problems sometime in the future)
It makes calibrating the printer a pain tho, your print quality will degrade, head unit adjust uses black and Lm to calibrate, with no Lm, you won't be able to correct it.