Rambam writes (Hil. T'shuva 3:15) that it is forbidden to utilize any entity as an intermediary between oneself and God (more accurately that one who does so is a heretic who loses his share in the world to come and is in the same category as polytheists):
חמישה הן הנקראין מינים: האומר שאין שם אלוה, ואין לעולם מנהיג; והאומר שיש שם מנהיג, אבל הם שניים או יתר; והאומר שיש שם ריבון אחד, אלא שהוא גוף ובעל תמונה; וכן האומר שאינו לבדו ראשון וצור לכול; וכן העובד אלוה זולתו, כדי להיות מליץ בינו ובין ריבון העולמים. כל אחד מחמישה אלו מין.
He writes this in his Mishnah commentary (Sanhedrin 10:1) as well:
מן המלאכים והכוכבים והגלגלים...אין עושין אותם אמצעים
That is one may not use angels or other celestial forces as intermediaries to God. (This is the 5th of his 13 principles of faith).
Would there be any difference between this and asking a human being to intercede on one's behalf to God?
(That is, do any authorities forbid the former, but permit the latter; difference, or forbid both; no difference).
I have seen an explanation quoted from Maharal that one is permitted to ask humans to intercede on his behalf since people are commanded to perform kindness with each other, but I found this explanation unconvincing and am looking for other views.
Additional evidence regarding the general prohibition to pray to angels:
Rabbi Yitzchak Lamprontti (17th-18th cent.) cites the aforementioned quotation from Perush HaMishnayot as a prohibition to pray to angels as intermediaries (here).
This is also clear in the famed "ani ma'amins" (a restatement of his Rambam's principles)
לוֹ לְבַדּוֹ רָאוּי לְהִתְפַּלֵּל, וְאֵין רָאוּי לְהִתְפַּלֵּל לְזוּלָתוֹ
Although Rambam's statements seems all inclusive, it is interesting that in neither of the aforementioned citations, or in his somewhat similar ruling in Hil. Avodah Zara (2:1), does he mention human intermediaries explicitly.