Generally I'm in favor of this, but I think two characters are too few, and the abbreviations are almost arbitrary in some places. It's great to be brief and remove clutter but it's bad to be cryptic. I'd be in favor of short but meaningful tags like [PARAMS] or [ISA] or even [SWITCH HEADER].

src/arch/isa_parser.py (Diff revision 3)

We might not want to do this. It could be confusing whether those messages are coming from scons or the parser.

Love it.  One thing I'd like to see would be to strip BUILD_DIR from the file name when printing.  I can do that in a separate changeset though if you prefer.

SConstruct (Diff revision 3)

I think that you should do this one the same way that you do the others with the getOutput function

src/SConscript (Diff revision 3)

perhaps it would be better to have a MakeAction() function that buries the *getOutput code inside and just does something like:
def MakeAction(func, string):
    if env['VERBOSE']:
        return Action(func)
    else:
        return Action(func, string)