yes, after a little research TinyMCE does have a spellcheck module and i got this to work in modx really easily within 15 minutes. so if anyone else needs to know ...
the spellcheck plugin can be downloaded here:
http://tinymce.moxiecode.com/download.php
and the wiki documentation for the spellchecker is here:
http://wiki.moxiecode.com/index.php/TinyMCE:Plugins/spellchecker
i followed the installation instructions on the above wiki page and then within modx configuration:
- set TinyMCE theme to custom
- added "spellchecker" to the end of the Custom Plugins line
- added "separator,spellchecker" to the end of row 1 custom buttons
that was it
note: i already had PSpell installed on my server and i only needed the default english library so i needed do no further configuration or installation.