You got any pics of the current setup? Might help for some ideas to catch the run off. Otherwise seems like it might be easier to just get some extra treys and swap them when full.
Coco tends to have a bigger problem with salt buildup compared to other hydroponic mediums. Sometimes giving water instead of a feed is not a bad idea, such as if one were to overfeed on accident. I wouldn't give it an actual flush and by flush I mean run a high volume of water through it. That I would never recommend. I do know people that do that in soil and have very successful grows. To me it just seems like they're creating more work for themselves. I usually call those type Fox Farmers. Whatever works for them I guess.
In most cases I think a lot of these hydro flushers may want to try using a more dilute feed. Obviously proper Ph is also very important but there is something a lot of growers overlook including myself at one time. Those damn piece of shit cheap Ph meters. They are not even worth using and should be illegal to sell. However the General Hydroponics PH Test Indicator solution is cheap and accurate. It does not give as much precision but it will easily get you in the proper range. I've tested several cheap and expensive meters side by side and pissed hundreds on Ph shit. I used to use the expensive meters but the probes are expensive and don't last forever, even when properly stored.
There's not much to show of the bottom of my tent, there are 2 plants from clone and 3 from seed each in an 8" pot which is sitting in a saucer. This was not how I planned it, I have an 18" like 30L barrel planter that I will be using for my next grow (which also will be my first time with promix hp), they are in such small pots because of space and I once had this silly idea of trying to do a SOG with just 6 plants lol.
I didn't anticipate how quickly the clones would respond to 12/12 vs. the ones from seed so it has been a bit of a balancing act propping the 3 from seed on some books and more aggressively LST the clones into the screen. I have these pot elevators that I plan to use with the 18" pot (but I don't have enough for the current setup). I will probably just go with the easy answer of shop-vac, because I have 0 vertical space to spare. The tent is only 5' tall.
I agree that if you overfeed or something that needs to be rectified. It has just been my experience that plants respond better to gradual changes in EC and pH of your nutrient solution. Rather then making a large change in one feeding, I would rather spread it out to 2 or 3 feedings. I have no scientific basis for this belief, it has just seemed like my plants respond better if I increase/decrease the pH or EC over more then one feeding.
I have a bluelabs ph meter, imo it was a waste of $100. Those cheapy ones are kinda crap, and there's nothing wrong with the bluelabs, it's just not worth $100 for the amount of care my lazy ass is supposed to put into it. pH paper is good to have around too, not for accurate measurement by any stretch, but it helps you to confirm the accuracy of your pH meter and it's quick. I try and keep a pretty close eye on my pH but to be honest this AN Sensi Bloom ('pH perfect') is working out pretty much as advertised (unbelieveable I know) and the pH of the solution without any ph up or down adjustment is about 5.9-6.0 with tap water (pretty hard like 290ppm caco3) with the pH of runoff never being more then like 0.2-0.3 units off of the 5.9-6.0 they were fed (which is good!).