Hi Ian,
I am sure this will not help you much but as workaround we don't use classes and specify style settings for each element (tables, cells, etc.) inside the manual message template:
<table style="...;...;">
<tr>
<td style="...;...;">
It's really not very intelligent but it works.
Besides it's only for manual message templates (sent manually using html editor). Common css definitions using classes are processed fine in all the other (automatic) message templates.
As well as you I would really like if html editor control could support (read, display and send) common stylesheet definitions but I haven't succeeded to make it working yet.
Regs
Gytis