I don't know if anyone else has tried this (and it probably won't work with your Magnum pump since the flow is always on), but I've had a reasonable degree of success with this manual siphon method:
1) Plunge the vacuum into the sand bed. You'll get a big plug of sand/detritus to start rising up the vacuum.
2) Pull the vacuum up just above the sand bed and immediately kink the hose to stop/slow the flow way down.
3) The heavier sand falls back down into the sand bed while the detritus will remain mostly suspended in the big tube.
4) Unkink the hose and VOILA! Detritus gets sucked out, sand stays in sand bed (for the most part).
5) Repeat until a) you're out of patience, b) you're out of detritus, or c) you're out of water.
I've read some suggest to only do one part of your sand bed at a time with this method so you don't devastate your whole bacterial population in one shot, but you do pull out a lot of junk and probably more so than just skimming the top of the sand bed. Anyway, just a vacuuming tip on RC that I've found to be helpful...
I have a ball valve on the inlet hose. I've been playing around with basically doing same thing you describe by controlling it in one hand while vacuuming with the other. The nice thing about the Magnum is that I never run out of water. My water changes are done automatically anyway so it's nice not to have to deal with any buckets to do this.
I plan on cleaning half the tank every week but I'm not really concerned about the bacteria. The sand is purely cosmetic. This isn't a DSB so there is no anoxic zone to disturb. I have plenty of live rock and DSBs in the sump so I'm not concerned about any meaningful change in my bacteria population. You would need a really strong vacuum to rip a significant amount of bacteria out anyway.
but this housing helped me take some really good and color accurate shots. I hate F'n around with things like cameras but this was really easy. Its amazing how much easier its is to take decent shots of a tank when you have the freedom to put the camera in the water! :lol: No clipping or photoshop, from tank to RC in less than 20 minutes!
"Freckles" the sailfin tang (Beth's name, I don't name fish) is in Velko's big tank and will probably be a foot long in a year. Check him out the next time your at Aquatic Treasures. 
