Don't know about Travelcards on their own, but I know you can do odd-period National Rail paper season tickets including outboundary Travelcard as I have done many times in the past. I believe you can do:-

One day (i.e. a single/return ticket or outboundary Day Travelcard)

One week

One month

Any period from one month to a day less than one year, the price for this is pro-rated from a monthly in some way (I don't know how many days it is based on in a month!) There will be no point doing this once you hit the price of an annual, of course.

One year

Neil