If there's something you can tie it to, sure. But none of my lids come standard with a hook, or anything protruding, to tie a string to.Next time maybe tie it to the release valve under lid? Anyone else dealt with this? Really pissed me off at losing some CO2 money down the drain. Or maybe just nix the marble?
I did buy 5 kegs from a brewery, they had welded a tab or nut onto the bottom of their lids. So those kegs/lids are being used when I know I'm going to be dry hopping in the keg.
I use a piece of very thin (picture hanging) stainless wire to keep the bag suspended. The wire is easier and faster to attach to that tab than fumbling with dental floss. Or I prepare the bag hanging from a spare lid that has a tab, and switch them out even faster while streaming CO2 into the headspace through the gas post at a decent rate.
BTW, I use a small handful of glass marbles, I doubt a single one has enough weight to prevent it from floating.