This is of questionable legality in my AO, based on reading case law. I thought about going this route, but I'm not sure it just wouldn't result in legal trouble, where a folder that was closed and drew on opening is clearly legal from the reading (as long as there is no button released spring used to jettison the blade).
The mid-line knife would be one of two carried. I usually carry two knives, typically a Clinch Pick or other fixed blade at basically the mid-line. The mid-line blade is a knife reserved primarily for emergency purposes. The second is a utility folder, used for all the other mundane tasks, but which can also be used as a defensive tool in the surreptitious role you describe above. Of course, one can use both for tasks and sometimes a reverse edge blade excels at tasks a normal blade doesn't, but that is really another thread unto itself.There are many sheath makers that will make an Emerson hook on their sheath. If one deems this reliable. I prefer a Knife I can palm closed so I can draw and prepare on the low down. Nothing is as consistent as the manual opening of a folder.