You could save the mass value in a skill and retrieve it through an impact_event:

action physic_ent
{
my.skill1 = 10; // mass value
...
phent_setmass(my, my.skill1, ph_box);
...
}

function static_ent_event
{
if(event_type == event_impact)
{
my.skill1 = you.skill1 // skill1 contains now the mass value of the physics entity
...do stuff...
}
}

action static_ent
{
my.enable_impact = on;
my.event = static_ent_event;
}

Edit: For your other problem (stucking with ph_poly), have you tried to increase "ph_check_distance"? But it´s only available at 6.40 and above.