How hard is it to build a promotor in a DNA that initiates DNA transcription when specific hormones are around
Promoters bind transcription factors (TFs) and RNA polymerase (or a related complex, in the case of eukaryotes). 

The substances you are interested in testing would need to bind to DNA in the same way as TFs and RNA polymerase, in order to trigger RNA synthesis (which you might measure with RNA-seq experiments). That would seem to severely limit what you can test the presence of.

In any case, if you could make that happen for your substances of interest, you would probably also want to make sure to avoid degradation effects of short-length RNA, by removing associated proteins.

https://en.m.wikipedia.org/wiki/Promoter_(genetics)

https://en.m.wikipedia.org/wiki/RNA-Seq

https://en.m.wikipedia.org/wiki/Small_RNA