cr housing units + evr improvements

Hi guys

Our idea is: We need to improve the shelter management (Configurable).
A shelter must be considered as a "container". Housing units must be defined within all shelters. A housing unit is part of a shelter and we can enroll evacuees in them.
The housing unit management guarantees a detailed management of shelters and evacuees.

We  don't care about the "type" of housing unit: 
If the shelter is a stadium -> housing unit is a tent
if the shelter is a hotel -> housing unit is a room etc
We care just what kind of evacuee could be accommodated.

I added improvements in evr module: added filters, added field descriptions, added report fields 

I really need help here.
During an evacuee registration we can register him in a shelter using the shelter registration tab.
We think It would be comfortable if we can enroll the evacuee  in a housing unit using the shelter registration tab.
At this time the housing unit drop-down field shows all the housing units defined in the related database table. 

In the shelter registration form we choose the shelter  where the evacuee is going to stay. I would like to choose the housing unit too but I would like to select it in a list containing just the housing units defined in the selected shelter.

Is it possible?