"only sparsely" is correct and usable in written English.
You can use it to describe a situation in which something is not abundant or concentrated. For example: "The crowd was only sparsely populated around midnight."
The republic is only sparsely inhabited.
By contrast, the Natuna Islands are only sparsely populated.
The streams are tree-lined, even in areas that are otherwise only sparsely forested.
The southern New Guinea plains are only sparsely populated by relatively mobile sago gatherers.
Up to now Cubism has been only sparsely represented at the Met.
But the equipment is expensive, and is therefore only sparsely deployed.
His difficulty in settling on a new path is only sparsely depicted, but Mr. Goldstein's grounded, spare acting is affecting.
At present, not unlike the frontier between Mexico and the United States, it is only sparsely fenced.
The coastal region was only sparsely settled at the time of the Thera eruption, with no identifiable city.
A dozen computers on the second floor had been provided for journalists to file reports; they were only sparsely occupied.
I'll be posting here only sparsely between now and the new year; I'll be tweeting, as usual, at @tnyfrontrow.
