icmp, folder redirection, and slow link detection

short version:

need a way to drop any icmp packets >2048bytes without third-party firewall
or switch

long version:

goal: trying to fool clients accessing local DC into thinking they are on a
slow link
issue 1: slow link detection uses 2048 byte icmp packets by default
idea: configure xp to use >2048 byte packets to locate local DC and get that
DC to drop 'em = voila, client will think it's on a slow link
issue 2 (unrelated): what happens if a client can't icmp the dc at all
(which is the scenario they will find themselves in if I can get what I want
here)? can they still authenticate or will they use cached credentials?

hmmn.