Yes, hold/toggle options are available for both crouching and aiming down sights.
I will try to find the link where I found this information.
EDIT: It's from H0RSE's http://www.fragworld.org/frag/community-blogs/brink-info-pt-4-controls-hud-and-everything-else.html
Default Xbox 360 controls:
RT = shoot
LT = aim
LB = sprint/SMART
RB = grenade
X = action/use (give ammo, give health, interact, etc.)
B = crouch
Y = switch weapons
A = jump
LS = buff self (give self ammo, health, etc.)
RS = melee
Up = Objective wheel
Left, Right, Down = class abilities
Invert Up/Down Aiming:
On/Off
Aim Sensitivity (Vertical)
X degrees/second
Aim Sensitivity (Horizontal)
X degrees/second
Aim Acceleration (Vertical)
Low, Medium, High
Aim Acceleration (Horizontal)
Low, Medium, High
Crouch - Hold/Toggle
IronSight - Hold/Toggle
Also if you are a fan of Killzone you will be happy to know (if you don't already) that Brink allows you to choose from a variety of control schemes, including "Zone of Killing" which I would assume is very close to Killzone's controls.
http://multiplayerblog.mtv.com/2011/03/14/brink-has-a-control-scheme-for-every-major-shooter-franchise/
There is also the option of fully mappable controls for both PC and consoles.
http://www.gamefaqs.com/boards/960407-brink/58882941/653508683 (Question 6)
Hope this answers your question.