While I'm not going to be able to help you with your original question I'll just spitball a thought or two on it.
1) Radon will escape from any soil matrix your testing to some degree.
2) If you have an air tight lid (thick HDPE) on for 20+ days before and during sample run the radon and it's decay products will not escape the container.
3) When it decays the resulting products will rain down on to your sediment sample. My main concern would be whether electro static interactions cause them to be attracted to the plastic container and then stick.
So while we seal ours in with epoxy, we do sometimes cut them open to get the seds for other analyis. We probably recover 3/4 of the the original mass but that will vary with your container and sed load.
My next suggestion I'm sure you thought of given your orginal question. Basically make some air tight gasket you could piston down into the container to seal in the seds in.
Out of idle curiosity what are your count times and who manufactured your gama specs? We have 2 coaxil germainium detectors by Ortec and we usually run samples for 23 or 47 hrs. 210Pb, 214Bi 226Ra, 137Cs and 40K is what we look for.
Its not often I get to talk to someone else running a Gamma spec so its nice to see what others are up to.