First the "main event" parts of NOx.

Dunno how they do it in Masschusetts, but in some states the timing is
part of the check -- if you successfully retarded static timing beyond
the ECM's ability to twiddle it on the fly (and the car still ran),
they'd either fail you for its being out of spec or insist on adjusting
it.

Cleaning up the EGR system was probably the biggest thing -- IF it's
working right.  That's the main way NOx emissions are cut down.  Timing
and mixture also affect NOx but you risk running up your hydrocarbons,
and you don't have a huge margin between the measurement and the limit
on those either (unless you want to replace the cat maybe?).  See for
instance
http://www.advanceautoparts.com/english/youcan/html/ccr/ccr20040501e1.html
http://www.asashop.org/autoinc/nov97/gas.htm
http://www.asashop.org/autoinc/dec97/egr.htm

As for MMT, I'm having trouble finding much authoritative information
on a one-shot use of an additive that features MMT.  This report from
Canada, where (in contrast to the US) MMT is a common component of
gasoline, suggests that MMT (at least at the levels commonly used in
gasoline itself) will probably reduce NOx levels in older
smog-controlled cars but might be a mixed blessing or a detriment in
Low Emissions vehicles and beyond:
http://www.aiamc.com/news/05_09_03_final_mmt_report.pdf
See also this paper, which suggests that, at least in some cars and
over a longish term, MMT can increase emissions and leave deposits in
the engines and contribute to cat plugging (not what you want to do if
you're marginal on HC, never mind its performance implications):
http://healthandcleanair.org/mmt/ford.pdf

A layman's summary of these issues may be found at
http://www.canadiandriver.com/articles/jk/040310.htm

MMT does come in for some discussion as a fuel-borne catalyst in
*diesel* engines, which AFAIK do not customarily have an exhaust
catalyst.

Now for a couple of practical matters:  how many shots do you get at
smog recheck before you  get declared to be a gross polluter or
something and enter some fresh hell; and how much does it cost you each
time?

Cheers,
--Joe