"purportedly only" is a correct and usable phrase in written English.
It is typically used to indicate that something is believed or claimed to be true, but there may be doubts or skepticism surrounding it. Example: The company's spokesperson purportedly only shared the official numbers, but many people suspect they are hiding the full extent of their profits.
Although Eggers's contribution is purportedly only to the look of the Believer, the DNA of McSweeney's was instantly traceable in its monthly spin-off when it was suddenly sprung on the world.
Best Buy is purportedly only going to be carrying the 40GB version.
The new and improved push recommendation feature purportedly only increases battery drain by about 0.7percentt per hour — or, "the equivalent of about a 20-minute game of Angry Birds" over the course of a day.
Although the ship was purportedly only carrying farming and forestry specialists, John Troy, then the Governor of Alaska, believed that the Aleutian Islands was a possible target for enemy navies and lobbied the federal government for military support.
The documents state: "In one instance a bundle of ballot papers purportedly containing only votes cast for the First Respondent was assembled by corrupt members of the counting staff and actually consisted mostly of votes for other candidates, with only the top few ballot papers containing votes for the First Respondent".
Although the Convention was purportedly intended only to revise the Articles, the intention of many of its proponents, chief among them James Madison of Virginia and Alexander Hamilton of New York, was to create a new government rather than fix the existing one.
Labour is purportedly just as enthusiastic about Sharkey's nomination.
Incidentally, Einstein purportedly once said, "The only reason for time is so that everything doesn't happen at once," proving that everybody, even the Time Man of the Century, longs to be included in Bartlett's "Familiar Quotations").
The BBC isn't the only purportedly unbiased media outlet to fall victim to false balance in its climate reporting.
In one review of 36 foreclosures at JPMorgan Chase, the bank was able to find documents explaining what the borrowers purportedly owed in only four cases.
Even founders owning common stock eventually realize that the billions of dollars' worth of wealth, they had purportedly created, was only on paper.
