NEB - Will T4 RNA Ligase 1 ligate single stranded DNA?

Yes. The reaction is slower than RNA/RNA or RNA/DNA. For example, primers can be ligated onto cDNA (2).
(2) Troutt, A. B., et al. (1992) Proc. Natl. Acad. Sci. USA 89. pp. 9823-9825.