Charlie,

You have just run into the primary issue with SU-8 - it is not easily
removable.  Unless a liftoff approach is used (see MicroChem website for
details), I've heard that a long soak in hot sulfuric acid may do it,
and that ashing has been successful. I don't know of any possible method
for wet etching silicon selectively (I'm assuming your are trying to
mask the material).