Naturally occurring non-binary HRT
PLEASE DON'T TRY THIS AT HOME! This is highly experimental and incredibly dangerous stuff.
This is not a Guide
We strive to provide non-biased, well cited, and accurate information, but this wiki is written by people who may or may not be professionals.
There is some evidence to suggest that certain phytoestrogens may act in a way analoguous to Selective Estrogen Receptor Modulators (SERMs). These phytoestrogens are reported to have a high affinity and agonism at ERβ, but a weak affinity and agonism at ERα. The point being that ERβ has a negative effect on mammary cell proliferation  thus it may be plausible to use such phytoestrogens if AFAB, or in conjunction with feminizing HRT to minimize breast growth while increasing other feminine features.
Naturally occuring ERβ agonists
Isoflavones in particular are able to displace 17β-Estradiol at ER, among them the isoflavone genistein is known to directly agonize ERβ. This class of phytoestrogens are found predominantly in soy products.
- Glycine max (Soy bean)
Many plants contain Apigenin which is known to preferentially bind to and activate ERβ 
Extended treatment with some non-synthetic sources of hormone replacement therapy is highly experimental and very little is known about it's long term effects. In the case of phyto- or xeno- hormones some are so obscure that it could be difficult to test for their blood concentration levels and thus proper regulation of dosage is impaired. In addition, dosage regulation is further impaired unless a proper extraction of the active constituent(s) is made, as the amount of substance in a given organism can vary batch to batch.
- Selective Estrogen Receptor Modulators and Phytoestrogens | http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2587438/
- Estrogen Receptor Beta Is A Negative Regulator Of Mammary Cell Proliferation | http://scholarworks.uvm.edu/graddis/259/
- Botanical modulation of menopausal symptoms: Mechanisms of action? | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3800090/