Just stumbled across this post by the HGVS Facebook page:

Even though intronic variants are represented with c. notations (c.31+2A>T, say), and cDNA notations need to be prefixed with the NM reference sequences, it would seem that intronic mutations need to be prefixed with the appropriate NG sequence. They're also recommending the Locus Reference Genomic sequences - and I plan to check out what these are.

https://www.facebook.com/HGVSmutnomen/posts/1013219148717242