I found a solution by adapting the answer to a different question which I found in Google Groups
It works with a custom ModelForm like so:
Create a new forms.py:
from django import forms
from models import Equipment
class EquipmentModelForm(forms.ModelForm):
class Meta:
model = Equipment
def __init__(self, *args, **kwargs):
forms.ModelForm.__init__(self, *args, **kwargs)
self.fields['equipment'].queryset = Equipment.avail.all()
Then in admin.py:
class SystemAdmin(admin.ModelAdmin):
form = EquipmentModelForm
filter_horizontal = ('equipment',)
Hope this helps someone else out sometime.