No one has mentioned the one thing that should be of concern when using an aircraft lav...sudden turbulence. I am not a nervous flyer and love to fly...but turbulence does come out of nowhere and one could be in the lav when it does. So I get in and get out as quickly as possible....I also wear shoes even if I have to stuff my feet with heavy socks (for comfort) into my shoes and clump down the aisle...I wouldn't put a lot of money on the aim factor being that good.
Edited to add a word about the source of the water in the lav. I know absolutely nothing about holding tanks on planes but I have been boating and around boaters all my life...the same tank provides water for the galley, shower, sinks and the head....I have never seen or heard anyone tell of a problem with lines mixed up allowing water to flow to anyplace other than intended...not that it couldn't happen.... just not heard of it....and an aircraft could be a whole different story....would like to know if it is.