BSL functions:

geomsky_init <max items> - enables the system and allocates memory for the specified amount of objects.
geomsky_nuke - destroys all items and frees the memory allocated for them

geomsky_add <name> <m3gm> <optional scale> <optional movescale>
geomsky_remove <name>

geomsky_set <name> <param name> <up to 3 values depending on param>

Params:
pos <x> <y> <z> - position relative to the camera. As this will render in the sky, small object close to the camera will still look pretty big and far away.
rot <x> <y> <z> - rotate in degrees. This will rotate, not set specific rotation, so setting this twice for 45 degrees will result in 90 degree rotation.
scale <value> - sets the scale
movescale <value> / movescale3 <x> <y> <z> - scales the movement on all or specific axis. 0 - static in the sky, 1 - moves at the same rate as everything in the level.
clearz <0/1> - if 1, then everything added after this item will draw on top of it and everything that was added before whether it farther away or not.
fog <start> <end> - same as gl_fog_start/gl_fog_end for this specific skybox item
useforcolor <0/1> - whether to use custom fog color or the one set in the level
fogcolor <r> <g> <b> - custom color to use for ths specific skybox item
visible <0/1> - sets whether this specific item is rendered or not