First - I fully understand your prevarication when faced with dealing with the head - but now GET ON WITH IT! lol
Second - Yes, this is something that I've heard of being done in the past - and it will help in the event of flooding with a couple of provisos:
1) You'll want to fit a very efficient strum box to the end of the pick-up tube in the bilge, or risk mincing your impeller when it sucks up some of the miscellaneous crud swilling around the bottom of your bilge (maybe yours is spick-and-span, but mine certainly isn't, and moreover since it's about 2'6" to 3' deep in an encapsulated keel, it's murder to get down into and clean!), fail to do this, and you'll risk having two crises instead of one!
2) Although there were mutterings about it on another thread, I would give serious thought to installing a bronze one-way flap valve in the vertical part of the bilge draining pipe - to guard against a failed y-valve gradually back-filling your bilge.