Most newer digital radios will not tune 87.7 - 87.9 is actually part of the FM broadcast band (channel 200) but there are only 2 stations licensed for it, as was noted earlier in this thread. From what I understand, no new FM licenses are being issued for 87.9. I don't know enough about LPTV rules to comment on the legality of WJMF-LP airing audio on 87.9 but I would hope they consulted with their broadcast attorney before doing this.
Something else to consider - is 87.9 considered part of the non-commercial FM band? If so, then no commercial advertisements can be aired.
RFB