A Reconstructionist minyan in Germantown is considering adding Bilhah and Zilpah to the text of the Amidah. Of course, this comes after adding the usual Fore Mothers (Sarah, Rivkah, Rachel, and Leah). They have a lengthy post with discussion both pro and con.
I have some problems with this proposal, aside from simple Orthodox "my way or the highway."
1. Slippery slope (this seems to have been intentional in many congregants' thinking). You start adding and there's no end to adding. The same is said for adding praises of Hashem - start adding and there's no end to adding, which is how we have the list of praises in Yishtabach - these are praises explicitly said by Moshe. It's also said in the Talmud at the beginning of Tr. Yoma, that one doesn't appoint a substitute wife for the Kohen Gadol, because the first might die and he needs a living wife to do the service - the mishna explicitly says if you do, "there is no end" to the substitutes you'd have to appoint.
2. National consensus. There has been a lot of regional variation in the texts of the Amidah, some still remains, but everybody (from the Talmud until the 1980s) has followed the consensus text of the first three paragraphs, which are laid out in the Talmud.
3. Theology (I-Thou). It seems to me that we include the Avot because the Torah gives us some clue about their varying relationships to God. What clue do we have about the varying relationships between the Imahot, let alone Z&B who are barely mentioned as brood mares, and God? Sarah & Rivka related to God as arbiter of disputes between themselves and their husbands over preferential treatment of the children. Rachel & Leah's relationships to God only come out of Midrash.
4. Textual. Well, really, we include the Avot because most of the Amidah is made up from verses or phrases from verses. Is there a verse "elokei Sarah, elokei Rivkah, elokei Rachel Leah Bilhah uZilpah"? Or even of each phrase separately?
Shoehorning the Imahot into the Amidah feels about as awkward as the love interests shoehorned into "The Hunt for Red October" (Clancy's publisher forced him into it) or the movie version of "Fantastic Voyage" (Asimov's novelization has no love interest, but I did learn a lot about biology from it).
Full disclosure: I've been teaching a series on the brachot of Shmoneh Esreh for the past year and a half, intermittently, when the rabbi is not present at seudah shlishit, at the Yavneh Minyan of Flatbush. I cover both interpretation (phrase by phrase and structural, via Baer, Gra, Netiv Binah, RY Emden, and others) and textual history (via Fleischer, and Luger's book on the Genizah texts of the Shmone Esreh). We're in the middle of the Blessing of the Righteous, and I plan to cover Al Hanisim during Chanukah. If you're in the area, come on by - mincha at 4 pm Saturdays for the next 3-4 weeks.