30 years ago, Mike Collett's outfit - Air Atlantique - did it with DC3s, fitted with tanks and spray booms and based at Exeter. Lots of sitting around and training flights, but at least one call out in anger to a huge slick off the South coast.
AA converted a Orion P3, some years later, and I witnessed a demo down Coventry runway for the benefit of the Min of Ag and Fish. The pilot flew it at about 40 ft, flat out, and sprayed the whole runway length with great accuracy. Impressive, but I don't believe anything came of it.
The B727 idea could work, I guess, but I would have thought there are better airframes for the job lying around in aircraft graveyards, if jets are under consideration.