MadcowsWho isn't?
MAX_MAP_CLIPNODES
There isn't any way to add more nodes or clipnodes to the bsp's (its already maxed out). However, at least clipnodes can be reduced with a bit of work.
When the maps are compiled all the 3d 'space' a player can get to is broken up into convex regions just like brushes are required to be, alot of them are extremely small or too small for a player, and if you put a CLIP brush over them they don't become clipnodes at all (well really there are still a few intersecting the brush on its surfaces, but the brush can be excluding dozens or hundreds of them at a time).
If the world has had a 'box' placed around it to prevent leaks, its probably causing several thousand (if not 10000+) extraneous nodes and clipnodes to be caused, not only wasting resources but will cause vis to work a lot harder than it needs to. An example map is available, demonstrating how it is possible to reduce the clipnodes in a map. Without the clip brush in place, the map requires over a hundred more clipnodes to define the player-accessable space.
Merle's new compiler also trims some excess unneed clipnodes, which means a little more you can build. Get it.