Heroic, and make martial arts II add the hammer weapon group.
You could even go so far as to make MA III for increasing an addl die in Paragon, possibly even adding the High Crit property at the same time.
This would make that whole path a whole lot more palatable. 3 Feats for a d8 hammer with the high crit property (at earliest 11th lv) that can't be taken away from you unless you're restrained. Yeah, I think that's reasonable, particularly since it won't ever be a magical weapon, unless you add some gloves to your campaign that add that trait to a fist weapon, and even then I think it still works.
Consider the following name changes though:
Pugilist (+2 prof, off-hand property), no prereq
Brawler (+1 die size, Hammer group), no prereq
Martial Artist (Paragon, +1 die size, High Crit), req both of the above
(EDITED the list above to reflect this paragraph and Wyld's note about off-hand below.)
I would even go so far as to change the benefits for Pugilist and Brawler (since Pugilist is more about talent and ability and Brawler would be more about raw power), and not require either as prereqs to the other, but still require both for Martial Artist.
Consider even adding an Epic Feat: Martial Artist Mastery for an additional die (not die size, making the fist weapon a 2d8). Though I'm not entirely sure that would be completely balanced, but considering it's epic level and doesn't allow for a magical version, it might be. Alternately, +1 die size and crit on 19 might be more balanced, particularly if you include those gloves that give magical properties and bonuses to a fist weapon.
I would also consider allowing the Paragon or Epic versions noted above to be allowed to be used with Sneak Attack. Gives the whole assassin feeling... or maybe something specific to the Shadow Assassin Paragon Path - not sure about that one.