This is in version 0.9.9.1
In my recent Nightmare game I was determined to clear the mortuary. I used the Nuclear BFG to help me by gibbing corpses. Eventually I killed everything but could not find the Angelic Armor. Corpses continued to raise and I killed them all again but still could not find the armor. I repeated this maybe two more times.
After that every time I killed the last remaining enemy it always generated an Angelic Armor. I ended up grabbing 4 before I got bored and left the Mortuary.
Here is the relevant part of my mortem:
-- Equipment -------------------------------------------------
[a] [ Armor ] Angelic Armor [7/7] (89%)
[b] [ Weapon ] modified plasma rifle (1d8)x6 [40/40] (P1)
[c] [ Boots ] modified plasteel boots [4/4] (100%) (A)
[d] [ Prepared ] modified BFG 9000 (11d8) [169/169] (B2P1)
-- Inventory -------------------------------------------------
[a] nuclear BFG 9000 (8d8) [40/40]
[b] modified red armor [4/4] (200%) (AB)
[c] Angelic Armor [7/7] (100%)
[d] Angelic Armor [7/7] (100%)
[e] Angelic Armor [7/7] (100%)
[f] power cell (x70)
[g] power cell (x70)
[h] power cell (x70)
Edit: This is running on Win XP