
I noticed in Deep Space Nine episode "The Search" that the Romulan, Sub-Commander T'Rul, seemed similar to Seska. The facial features and voice sounded very much like Seska's actually. I think if Seska were changed into a Romulan she could pass herself off as T'Rul in any event.
The main issue with this theory is that Voyager episode "Caretaker" occurs on stardate 48315.6 and DS9 episode "The Search" occurs on stardate 48212.4. That does not give her much time between going from an undercover Romulan to an undercover Maquis. Plus Janeway says in episode "State of Flux" that Seska was in the Maquis for the last 2 years.
However we've seen in episodes of Star Trek that performing a species change operation does not take very long at all like Dukat from a Cardassian to a Bajorian or Kirk from a Human to a Romulan does not take long. It's conceivable that she also had time to work a second job as the Romulan Sub-Commander T'Rul. Or is it just a coincidence that they both have similar features and both happened to be in the same area at the same time?