There's a few things that affect blackholing. The first is top-level items, as already mentioned (as well as other players and NPCs in the area.) The second is the connection speed of the player (its pretty rare on any sort of broadband connection, but dial-up players have the problem far more often.) The third cause is the one that tends to affect WoD much more than other servers (as Fitz mentions playing on other POL shards that don't have the problem much): the items that you're carrying when you try to log in.
Simply put, every item that you're carrying counts as a top-level item for you (meaning that it needs to be loaded when you log in.) Since most UO shards have rather strict weight limits, they don't have to worry about this too much. With WoD's wide-open carrying capacity, though, a lot of people tend to carry around backpacks full of backpacks full of junk (there's currently 107 characters with more than 250 items, and 17 with more than 500.)
This is actually a pretty good argument for why WoD needs stricter weight limits. But that's a different subject.
Getting back to the original subject, though (getting more lockdowns), this is something that will almost certainly happen at some point, though its not going to be free. My current plan is to give houses levels. When you buy a house, it would start out as a level 0, and have the same number of lockdowns as it does now. Houses could be upgraded to a maximum of 2*HV (so a HV 4 house could go to level
, and each level would give an extra 5 lockdowns (so 50 extra lockdowns for a fully upgraded HV 5 house.) The cost to upgrade a house would be 10k times the new level (so 10k to get to level 1, another 20k to upgrade to level 2, 30k to go to level 3, etc.)
This will probably happen eventually, but as I said, it may have to wait a bit until some of the current problems with people blackholing is fixed (which probably means waiting until POL 0.96, when I'll be able to make more stringent limits to carrying capacity.)