If MOL wants to do this he should allow passengers to buy loo credits in advance of flying (on the website when booking). If one credit in each direction were the "default" option many would end up buying them without realising. Those who chose to opt out could get a pop-up asking them "Are you sure you won't need to use the toilet?", much the same as happens now if you opt not to take Ryanair's insurance. Anxious passengers who are worried that they might need to "go", passengers with prostate problems etc. would be a lucrative source of additional income. Paying in advance would, of course, be cheaper than paying for "going" when onboard. Passengers who pay could be given a PIN to use. This would have the benefit of the airline not having to collect coins from toilet doors. (Moreover, having to remove coins once the coin boxes became full might have an adverse impact on tight turn-round times.) People who pay in advance but do not need to "go" would be like the airline's no-shows. There would be a problem ensuring that these passengers did not give their PIN to someone else. I'm not sure how this would be policed. Perhaps cabin staff could ensure that the person using the PIN was the person named on the confirmation printout. After all, all passengers will have photo ID on them.
Taking the idea further, since Ryanair's aircraft have three toilets, one could be designated as a "premium" loo, with upgraded facilities (and fee) to match.
The more I think about this the more promising it looks.