In partial answer to my own question, the answer is no, the canal was intended to aid rapid flooding of Romney Marshes, thought to the most likely landing point for any French invasion. With regards to its use as a defensive feature:
"The excavated soil would be piled on to the northern bank to make a parapet, behind which troops could be positioned and moved out of sight of the enemy. The canal would also have ‘kinks’ to allow enfilading fire along the length of the canal, if the enemy attempted to cross it."
"The total length of the canal would be 28 miles, of which 22.5 miles had to be dug. It was estimated that it would be completed by June 1805 and cost £200,000."
"Pitt met with the landowners on October 24 at the New Hall, Dymchurch and explained that the canal would not only help to defend their country but would be a major drainage system for the winter, and a reservoir for the summer and would greatly improve conditions on the Marsh. They were persuaded. Pitt was popular with the locals, who referred to the canal as ‘Mr Pitt’s Ditch’."
The contractors proved venal and incompetent in the face of adverse weather and flooding. Rennie, the engineer, proved unequal to the task of managing the project. When, after six months work had stalled with only six miles dug, the job was put in the hands of the Quartermaster General's department, but nonetheless was not completed until 1809 (only £34,000 over budget..... ).http://www.royalmilitarycanal.com/pages/history.asp