halacha based on nevuah - lo ba'shamayim hi

The gemara (Meg 3 and other places) derives from the last pasuk in our parsha,  אֵלֶּה הַמִּצְוֺת אֲשֶׁר צִוָּה ה׳ אֶת מֹשֶׁה אֶל בְּנֵי יִשְׂרָאֵל בְּהַר סִינָי, that a navi is not permitted to introduce a new din or mitzvah beyond what was given at mattan Torah at Sinai.  

Chasam Sofer asks: what then was the hava amina of R' Eliezer in trying to prove he was correct in the famous tanur shel achna'i sugya by calling on a bas kol to affirm his position?  Lo ba'shamayim hi -- you cannot create halacha from nevuah.

I am a bit befuddled by the question.  The assumption that we ignore a bas kol because lo ba'shamayim hi is not so simple.  The gemara (Eiruvin 13b) tells us that it was on the basis of a bas kol that we pasken like beis hillel over beis shamai.   Tos discusses why we rely on the bas kol in this case but not with respect to tanur she achna'i and offers two answers:

 וי"ל משום דבת קול דר"א לא יצאה אלא לכבודו שאמר מן השמים יוכיחו כדאמר התם

1) The bas kol came out in support of R' Eliezer because R' Eliezer called upon it to do so, so it was bound to be mechabeid his wishes.  The bas kol came out in support of hillel without being prompted to do so.

 א"נ בההיא בת קול שהיתה כנגד רבים דרבנן הוו רובא (ב) דודאי אין הלכה אבל כבת קול דב"ה קי"ל משום דהוו ב"ה רובא אלא דב"ש מחדדי טפי

2) R' Eliezer was opposed by the majority; beis hillel was in the majority.  The reason the bas kol was needed was because beis shamai were brighter/sharper, and so we have conflicting halachic rules -- follow majority vs. follow the sharpness.    

According to the first answer of Tos, it seems like a bas kol does carry weight so long as it is an unprompted revelation.  The second view limits the role of the bas kol to cases where the halachic process does not leave us with a clear resolution, but it still has a role.  

The Rambam (Yesodei haTorah 9:1) sounds like he categorically rejects any use of bas kol to determine halacha:

ונאמר לא בשמים היא. הא למדת שאין נביא רשאי לחדש דבר מעתה. לפיכך אם יעמוד איש בין מן האומות בין מישראל ויעשה אות ומופת ויאמר שה' שלחו להוסיף מצוה או לגרוע מצוה או לפרש במצוה מן המצות פירוש שלא שמענו ממשה. או שאמר שאותן המצות שנצטוו בהן ישראל אינן לעולם ולדורי דורות אלא מצות לפי זמן היו. הרי זה נביא שקר שהרי בא להכחיש נבואתו של משה

Be that as it may, just because Tos wants to have its cake and eat it and accept the bas kol with respect to paskening like beis hillel but reject it when it comes to tanur shel achna'i doesn't mean R' Eliezer has to agree.  R' Eliezer b'pashtus would tell you that it's all or nothing -- what's good for the goose, i.e. affirming beis hillel, is good for the gander, affirming his own view.  

A second point: if C"S is correct, Tos should have asked a simpler question.  How could the bas kol affirm the halacha is like beis hillel when we have a rule that אין הנביא רשאי לחדש דבר מעתה  -- no need to mix in the sugya of tanur shel achna'i.  It sounds like the Tos rejection of the bas kol is only predicated on that sugya, not the derasha from our parsha.  Also worth noting that the Rambam does not cite the derasha either (see Lechem Mishne).  

Confusing.

For a general overview of what role nevuah or ruach ha'kodesh (e.g. "sod Hashem l'yireiav," as we saw in the Raavad a few weeks ago) can play in halacha, see R' Kook's comments at the end of Mishpat Kohen herehere.