There's one report of an accidental exposure that led to 2 deaths. Enema is supposed to be sodium (I think) phosphate and these two had sodium nitrite solution at 15 g/liter strength. I found this notable as most commercial enemas only use 4 oz. Of fluid so death occurred with a less than 2 gm exposure. It seems to require less than the amount taken orally, but it's only 2 cases so it's tough to draw a big conclusion on so little data.
Certainly it could work if you have proper dosage and could keep the solution in long enough to be absorbed.