Can the [tex] tag support arbitary TeX packages?

Unfortunately, no. I wrote this module originally so that those of us on shared hosting could use mathematical equations in a format similar to LaTeX, but the actual rendering is done using a Mimetex binary, created by John Forkosh.

However the Drutex module, when it is updated for 5.0 should be able to do this because it actually interfaces with LaTeX and LaTeX can use chemical formulas by the use of \usepackage{ochem}. Talk to this module maintainer about this possibility. The issue with the Drutex module, and it is a minor one for many people, is that it requires LaTex, Dvips, Postscript, and ImageMagick all be installed on your server (which is not the case for many shared hosting environments).

Dave