Hi Manders
See what you are doing now, in which case what you propose will work, i've never used a system like it but there are people (I think even in this forum, but cant find the link) who have created evaporative coolers with pumps and wetted media but not gone the extra stage and used a large reservoir of water as a heat sink. The exchanger design and air volumes will need thinking about. I presume you intending to speed control the fan on humidity rather than temperature, just allowing the heat sink to dissipate its heat by conduction to the surroundings.
Take your point on the gravel mix, I was just thinking of the mass rather than the thermal properties, apologies.
cheers
Steve