So all we need is the initialisation, and probably a common search routine for the convenience of plugins (although that isn't absolutely needed). The smaller the patch the more likely someone has time to test it and commit it.
I would vote for having utility functions that manages a list of available markers instead of letting plugins manually find those that are marked with SC_MARK_AVAILABLE.
I can imagine a situation when some plugin set its markers for several documents but didn't do it for others, so when another plugin tries to set it's own markers it came to that markers idenifiers for the same markers are different in different documents that means it's another job to keep track of it.
having such functions as utils_ui_get_marker(), utils_ui_free_marker() seems more clear and robust.
Regards, Alex