Here's a couple suggestions from my old Techtalk archives. I've not tried these and cannot vouch for them. But it's a variety of opinions. Take what you can.
"If you are worried about the hops sitting in the beer too long, you can simply tie the hop bag to the pick up tube. Then as the beer level lowers, the hops remain suspended over the beer. This is ideal for those that drink slow and/or don't like over hopping. (I don't know any of these types, but hear there are some out there ;-)"
"My method for keeping the hops bag down in the beer is to take a small cable tie, push the pointed end through the top of the muslin hops bag, start the tie's end into the receiver part, making a loop, then as you are inserting the dip tube into the empty keg, allow the tube to go through the tie loop, push the tube down several inches, and then securely tighten the cable tie around the dip tube, pushing the tube all the way in and connecting the liquid-out valve. I've done this quite a few times, and it works well."
"Many of us dry hop in kegs. I use a float and a coarse weave big hop bag, since under pressure, the hops will sink, and they (or the hop bag) can clog the "beer out" tube. A sanitized old While Labs yeast tube works great. I use homegrown whole hops.
Don't leave the hops in too long, or you'll get vegetable flavors. About 2 weeks is good, then sanitize your hand and the top, bleed the pressure off, open 'er up, pull the hops out, close it back up, and pressurize it again. I like to squeeze that hop bag into a pint glass, and OD on a few ounces of super hopped up ale.
Another fun route is to put a big hand full into a 2 liter bottle, with about a liter of your brew, and carbonate it for a while with a carbonator. This works well in as little as a few hours. It's about like a Randall. Have fun!"
"Steve asks about keg hopping. I've been experimenting quite a bit with that recently and I've got a few tips. First, you'll need to weigh down your hop bag. Otherwise if it is floating on the top it will greatly decrease the surface area available for the beer to pick up CO2. This can result in longer carbonation times. Ideally you would also want to find a way to keep it up above your dip tube inlet, or else you risk a clog. Suspending it somewhere several inches above the bottom would be best. Dry hopping works best at normal fermentation temperatures, so when cold it would need more contact time for the same effect. I don't believe the carbonation should make a difference, but maybe someone else will have an opinion on that.
Be careful with your hop selection if you plan to leave them in the beer until the keg is drained. Some hops will develop a grassy or catty character during extended contact with the beer. I've found Cascade to be one of those. I'd suggest letting this batch go as-is, but drink it within a few weeks. If you notice off flavors towards the end then next time either change your hops or stick with the secondary for dry hopping."
"Over the past few days we've seen a number of recommendations for dry hopping in the keg... weight the bag down, float the bag, or neither is just one example.
It is important to remember that in homebrewing there are very few absolutes, and I urge Steve, the original poster, to take all of our recommendations as merely a starting place, and experiment on your own to learn what works best for you.
Personally, I find it necessary to neither float nor sink the hops. I only use leaf hops in a keg, and either put them in loose with a surescreen on the dip tube, or put the hops in a nylon bag. I have never had a clogged dip tube.
I have also never experienced the effects of the commonly cautioned "Don't dry hop too long or the beer will get grassy." I find the OPPOSITE is true... for the first several days, a woody/grassy character exists, and once that subsides, the dry hop character only gets more and more refined as time goes on. I've kept beer on hops for up to 8-10 weeks without ill effects, and have not found that differences in temperature affect this. This is why it is important for you to learn what works for your system and your taste--not just for dry hopping but for all elements of brewing which don't have hard science behind it! "
Again, these are suggestions from others and may be conflicting but you can get a sense of what might work.