Marc,
it's not actually missing support for long or polynomial division - in your example code \def is in the list of blacklisted tex tags - see function tex_sanitize_formula($texexp) in filter/tex/lib.php and
$tex_blacklist = array(
'include','command','loop','repeat','open','toks','output',
'input','catcode','name','^^',
'\def','\edef','\gdef','\xdef',
'\every','\errhelp','\errorstopmode','\scrollmode','\nonstopmode',
'\batchmode','\read','\write','csname','\newhelp','\uppercase',
'\lowercase','\relax','\aftergroup',
'\afterassignment','\expandafter','\noexpand','\special',
'\let', '\futurelet','\else','\fi','\chardef','\makeatletter','\afterground',
'\noexpand','\line','\mathcode','\item','\section','\mbox','\declarerobustcommand'
);
This is necessary here for security reasons... not meaning that all of these TeX tags are vulnerable but allowing free use of them may cause some "unexpected issues".