The WPA rules committee took the "nurse safety" rule out of straight pool for 2008 because the rule was immensely complicated and confusing. And not really needed. It was replaced with a stalemate rule, so that if no progress is being made, you lag for a new break.
A related situation came up in one of my league matches yesterday. The cue ball was nearly on the cushion and frozen or almost frozen to a ball that was out more and in turn was nearly frozen to a third ball. We must have played six or seven safes each in that situation but it was clear that some progress was being made as the cue ball kept getting closer to a pocket and the object balls were being moved away from the cushion.