Moodle supports TeX filter to write complex expressions. However the use of TeX expressions is highly difficult and beyond general public usage.
However, almost anybody need to write some alphas o betas here or there. Or writing a mathematical operator, or some Arrows to point something. Nowadays text processors (MS-word and OpenOffice, for instace) have an "insert symbol" feature that allows to add any special symbol that cannot be accessed by keyboard typing. The full utf-8 range of symbols is available.
However, the HTMLarea editor only displays a reduced set of special characters in the insert special character" dialog. This patch is intended to add support for all other symbol blocks existing in the utf-8 encoding: the full 154 blocks with chars from U+0000 to U+10FFFF. Now this dialog will display the full range of unicode utf-8 characters supported by the web broser/font combination. A combo-box allows to choose the unicode block to show within UCS: Basic Latin, Greek, Math operators an so on. Even Arabic, gujarati, katakana and all others blocks, if desired.
To avoid making that selector too clumsy I have added a visibility setting that controls which of the 154 unicode blocks of characters are available. This could be added as as site setting in the admin tree (no done yet).
The attachment is tested in 1.9 HEAD, but should work as well in 1.7 and 1.8