This works as long as your form is always on that exact URL. An alternative, if you want your form to be more flexible, is to use arg(1) as a 'page argument' in the hook_menu definition and then add it as a variable in your custommod_form_name($var_here) function declaration.
Brilliant--you answered my question just before I wrote it. I see that if I use:
array('addciviparticipant_form',arg(1)),
for my 'callback arguments' then it indeed works as you say. But in fact just
array('addciviparticipant_form'),
also works. :)
The only one that FAILS is:
'addciviparticipant_form'
Thank you!!