Anyway, the bottom line for me - as already said - is to find a system which is fair and does not make anybody unhappy. This is why I tried to depict three main possibilities and list their pros and cons (and indeed many of these pros and cons have been re-stressed by several of you).
I am not particularly attached to any system. The reason why I suggested the "a posteriori" system is that it looked like few people seemed (at least to me) to be afraid - sorry for the bluntness - of being "cheated" by the system/by the others, with (possible) future repercussions on games, but also more importantly on friendships etc.. Hence the attempt to suggest something that would be really difficult to find unfair!
If the "a posteriori" system were chosen, as said, I would volunteer to keep the bookkeeping, which would not entail any particular burden beyond what I already do for the league (so no worries about that). Plus, there would not be any worries related to "forgetting" to pay etc., as the (very few: 2 per hyear? 4 per year?) sessions where people have to bring some money to "equalize" would be announced well in advance, so that players can come "prepared".
But once again, I have nothing against the jar system, which may be in fact easier to implement. However it sounds a bit arbitrary to me, plus it may incur into issues such as the following. Suppose (a situation not that unlikely!) that for instance a 4-player table is made in Brussels with me (local), Dennis (by train/car), Bram (by car) and Luc-Olivier (by car): how are my 5 or 7 or even 10 euros supposed to contribute to all the expenses of all the travellers?!?
Emiliano "Sans Terre" - Wandering Player - Prince of Brussels