If you are using the SaveAsWeb feature, then all of the visible shape
data/custom properties are exported into the data.xml file ... so, you could
write JavaScript functions (or whatever) to edit this file, but then you
would need to write something more code to merge these chnages back into the
original Visio file ... and you would obviously not have any graphical
changes happening in the web page. Not for the faint hearted!