Is it possible? The thought of wasting a weekend driving to Phnom Penh or even doing a visa run.....
I should have done it in Sihanoukville...
Anyone know of any agents in the area?
I don't know how well known you are in the area or if you have a local friend who can assist but you could try the police station.
Most police stations in bigger towns have an immigration officer and the are usually helpful when it comes to visa. Passport still has to go to Phnom Penh but they will send it for you and charge you $5-10 on top of normal rate for visa. If you consider the alternatives it's a bargain.
Best to check with a local about trustworthiness of officer involved. My experience is positive (in another cambodian town) but it most likely differs from place to place