The closest thing we probably have is the k-Nearest Neighbors option in classify.seqs. You could do it in an alignment-based approach to get the most accurate match to what you want. Alternatively, you could use kmers. If I remember right, seqmatch is kmer based, right? By changing the value of "k" you can select how many sequences you want for the consensus. The distance-based approach will output the name of the closest sequence and the similarity between it and your sequence. To mask out the N's, you could always use filter.seqs(trump=N), but that would remove that column from every sequence. Let us know what you think...
Pat