Thanks for taking a look! It will be really nice if you can get TMathTex working for PDF and/or SVG. Unfortunately, I really do need a vector PDF as it’s a requirement for the journal, so the raster-to-PDF idea won’t work.
I managed to get something reasonable by saving the canvas as a .tex, embedding that into an otherwise empty LaTeX file and compiling that file to a PDF. It’s a few extra steps, but it seems to preserve both histogram transparency and TeX characters. Because I know others have had this issue and might be interested, the frame file I used to compile contains simply (for a canvas printed to c.tex)
\documentclass[tikz]{standalone}
\usepackage{amsmath}
\usepackage{standalone}
\usetikzlibrary{patterns}
\usetikzlibrary{plotmarks}
\begin{document}
\input{c.tex}
\end{document}
and is compiled by pdflatex as long as you have the standalone package (if you don’t, you just have to import the tikz packages and set the pagesize and margins yourself).
It would be cool to have an option in ROOT to include the few lines at the beginning and end to make the .tex compilable out-of-the-box, but I think this is an easy enough workaround.