I think this is an area where a bit of common sense must be applied. Would you feel comfortable getting into a manhole if you had to crawl under a trailer and didn't have room to set up a ladder or a tripod for confined spaces rescue?
Charlie is absolutely right, of course. In addition, when "egress" is an issue the focus moves to the Life Safety Code , OSHA and similar documents. It really is no longer an "electrical" issue. Check the following site and do searchs on "confined space(s)" and "means of egress."