with the foam out of it, and a few large rocks, and no current in the area you sink it ..... I'd say it would stay down. You would, of course, have to tow it and all the other materials to the lake - launch it and your "other" boat - tow it to a specific spot - transfer all the "weight" and "condo material" onto it - then compromise the hull in such a way that it would sink straight down. You could cut some spots out in the flooring and place PVC (in buckets of concrete) or whatever in there ... and in any of the other "open" holes already in the cap.
What you might consider doing -- is ask the Fish & Wildlife Dept ... if it's ok to do it, and if so - would they help ? If it's allowed, they may want you to tell them where it's dropped (or they'd GPS it, if they helped). If they won't allow it, you'd probably be better off selling it for whatever you could get out of it (even if boat is worthless, trailer may fetch some $$).
Just some food for thought ............ luck2ya ........ cp