Personally I've never seen that big of a problem with modreqing for fluids. Granted I've only been playing since S rev 15, but in that time I have built many grinders, auto farms, and tons of other builds that needed complex water flow. I can never remember having to wait more than 18 hours at worst, and usually it was under 2 hours. definitely not uncommon to have them filled in a matter of minutes. I have played at all hours of the day (unemployment can really screw with my sleep schedule) and still there was usually a mod or admin who would hop on and catch the easy modreqs. If i got too impatient I was usually able to find water nearby that i could use to avoid a modreq altogether.
All that being said, since I was made a mod, I have loved being able to place water instantly. It does make the game-play more seamless. So from a player POV I would really like to see it place-able.
From a mod point of view, I don't mind the water flow modreqs--actually I enjoy them. It seems to me like the other staff does as well. If there is more than one staff playing when one pops up it is frequently a race to see who can claim it first. (damn you buchanman.) It's a fun interaction with the player base that I enjoy. While I don't usually stick around the whole time, if I notice the player is working on a grinder i'll usually tell em to feel free to msg me with any more so i can get on it ASAP. (I would like to caution against suggesting that players /msg a mod to draw attention to a modreq as that sets a dangerous precedent of bugging mods to get modreq completed, it is a volunteer position and I would hate to see mods getting burned out)
I would much rather spend 2 minutes completing a water flow than spend an eternity and a half trying to clean up a huge lava/cobble/water mess that a player was able to build just by alternating buckets of water and lava. (I've never had to clean that up, but Kitcatbar didn't seem to think too highly of me when she had to clean it up on a chaos server a few revs back... oops) A player only needs 6 iron for that, pretty easy to find in almost any surface cave. I definitely think it would be worth a trial run for a week to see if it is abused especially with a game mod in place to prevent rail and redstone grief (would that mod destroy obsidian generators?) and i would agree that it shouldn't be tried mid-rev. I think the perks that the playerbase stands to gain is worth the added risk of griefing. BUT during that I would suggest that we keep a very close eye out and if it does start to become an issue, revert back to the current system.