There has been some work with psoriasis and photodynamic therapy and indeed it can be helpful. Since seborrheic dermatitis is a mini-me of psoriasis, PDT probably benefits this condition too although I know of know papers demonstrating this. ( Seborrhea being a major symptom of seborrheic dermatitis.)
There are so many better and less expensive treatments for this problem than PDT, I cannot see a reason for its use here. Regular UVB light for one if you want a light based treatment. A host of tar-based, zinc-based, sulfur-based, salicylic acid-based shampoos. Topical steroid or a new product called Promiseb work, although the former is not a long term solution. Since many cases of seb derm are due to a yeast-like organism called Malassezia Furfura shampoos or topicals such as Loprox shampoo, Nizoral Shampoo or Naftin cream are effective. Cacineurin inhibitors work too. The old standby of selenium sulfide found in Selsun Blue or prescription strength Selsun can still be summoned.
General advice would be to shampoo more frequently and leave the shampoo on longer. Discontinue the use of hair sprays and pomades.